The question is simple. When should I decide that I went to far over the deadly encounter threshold? When I have exceeded the deadly encounter threshold + itself * 50%? When I exceeded the deadly encounter threshold by itself + easy encounter threshold?

My problem comes with the following example. I have a party with the following encounter thresholds:
Easy: 75 xp
Medium: 150 xp
Hard: 225 xp
Deadly: 400 xp

If I want a hard encounter i can put 6 bandits with an adjusted xp of 375.
But if i want a deadly encounter i have to put 7 bandits with an adjusted xp of 525.

My problem is, is this to much? how do i know when i have crossed the line?
I know this question seems trivial, but, I like to have give my players, sometimes, really hard encounters, but, I don't want to crush them with an unfair combat that kills their beloved characters (without giving them chance to fight back).

Thanks in advance to any help.

I think there is some mistake in your calculation.

1 Bandit is 25 XP, challenge 1/8.

6 * 25 XP = 150 XP but because there is 6 monsters multiplier is 2. Making total challenge rating of 300.

Like this:

6 x Bandit (Challenge 1/8, 25 XP)

= 150 XP

Challenge rating: 150 x 2 = 300

Multipliers:

1 monster = 1

2 monsters = 1,5

3-6 monsters = 2

7-10 monsters = 2,5

How many characters you have in party and what are their levels?

The issue here is that the CR/Adjusted XP calculations are done in a white room setting. It does not take into account the PC's gear, cohesion, the terrain, or any other factor that goes into the fight. If you go by the numbers you are correct to assume that 7 Bandits are a deadly encounter. If you play the bandits with strong tactics, terrain advantage, and working like a well oiled militia, you could TPK your players with 5. All this before you take into account the randomness of the dice rolls where a Nat20 could change the tone of a fight in a heartbeat. That CR/Adjusted XP function is a guideline to get you started, the rest is done by you playing and adjusting encounters based on how well your players do. It's a learning curve for both the players and the DM.  

Two stories to emphasize my point:

Five level five PCs, fully rested, end up engaged in combat with 6 goblins, by all accounts this should be an easy fight. The trick to this combat is that the players were at the bottom of a sinkhole that was about 70ft deep. They'd rested in a depression in the wall of the sinkhole under a rocky outcropping. When the encounter began the goblins had height advantage, rocky outcroppings ranging from 10 - 60 feet above them and the goblins took advantage of this. They would fire their shortbows and retreat back against the wall of the sinkhole, this would give them a cover bonus. The PCs eventually beat them, however, two of the players were close to unconscious, one had blown a couple high level spells, and the group knew that their combat probably alerted any other denizens of the area. What is considered an Easy/Medium encounter was made very difficult due to using tactics and strategy.

Five level eight PCs, fully rested, ended up engaged in combat against a Kua-Toa Archpriest, a Hydra, a giant crab, and a giant crocodile. The creatures got a surprise round on the players due to a mishap on a player's part. Even with that, the players did 175 points of damage to the Hydra, no crits, on their first round, killed the hydra in 3 rounds, destroyed the giant crab in one round, and sent the Kua-Toa and giant crocodile running for the deep end of the lake they were by. In the following rounds they harassed the crocodile by casting water walk on it, and eventually killed it. The Kua-Toa used it's advantageous position underwater to make the pcs lives miserable, but it didn't amount to much in the long run. The adjusted XP of this encounter was 9,100 XP over the Deadly rating.

(Note: I do light magic in my games. The first party had two +1 weapons in their group, the second party had every character with a +1 weapon.)

In this case it was one character at level 3.
But it was just an example.

Yes I know power ratings are only guidelines and that there are a lot of random/non-random factors that changue a lot the nature of the encounter.

Still, I would like to reduce my chances of creating completly unfair encounters by following as much as possible those guidelines.

Another thing, how do you play with the monsters.

You adapt their tactic to their lore (the monsters one)?

You do what would be better in appearence (attack the players with martial weapons)?

You do what is better (attack the low armor players)?

I play a combination of Lore, personality and tactical intelligence.

A goblin is going to be terrified of adventurers if it's just him and a buddy. They'll run, hide, or surrender before fighting to the death. However a group of goblins, generally outnumbering the adventurers, or on their own turf, will fight. They'll use the terrain to their advantage, since this is their home turf, to get the drop on the adventurers in any way they can. They'll use guerrilla tactics since they can't compete toe to toe. When the fight starts to go bad for them, they'll run, hide, surrender, or lead the party into a trap. 

Bandits will vary depending on their goals. A band of cutthroats who have no home, trying to survive, will try to get the drop on unwary travelers. They'll use traps, subterfuge and other underhanded techniques to catch their prey. If they find they've bitten off more then they can chew, they'll call for reinforcements, run away luring the targets to a trap or larger contingency. If they're under the pay of someone, they'll do everything in their power to succeed, however if they're not afraid of repercussions from their employer, and the fight goes bad, they'll turn and run, surrender, or negotiate.

Creatures like Hobgoblins will be bloodthirsty, using their underlings as fodder to wear down their prey. When they are out of fodder, or their prey is weakened, then they'll join the fray. They may stay back and fire at ranged, they may have multiple groups of underlings attacking from different directions, they are tactical creatures. If they see the fight is going poorly, they'll send all of their fodder into the fight and use that as a distraction to get out of harms way. 

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As to targeting, I adjust based on many factors, and it all depends on the situation and creatures. Almost every time I'll have them target whoever hit them hardest. Assuming that no one has hit the creature, I'll have it choose whichever character is closest. If the combat is with a recurring group, they'll target the threats they remember, or may have been told about. As well, I almost never target a downed player, unless it's an unfortunate AoE type effect. 

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All of these types of thought processes help in adjusting combat on the fly as well. I can gauge if the fight is too easy/hard for the party and start adjusting numbers. Instead of 8 opponents fighting as hard as they can, if the fight is too tough for the group, I have a couple run away, reducing the number of opponents. This creates the ability to have the players run into consequences for the escaped opponents, but doesn't force a TPK on the players. If the fight is going too easy, I'll still have the two opponents run, but they'll return a couple rounds later with a few reinforcements to up the difficulty. Even as such, if the players are hell-bent on fighting to the death, I have no problem with using the "non-lethal" damage choice, when applicable, to turn a TPK into something a bit more interesting. 

Depends on the monster. I usually have it start by attacking the martials and spreading its attacks around. If a spellcaster / squishy starts to get really annoying through, it will change to focus them. They also don't usually hit characters that are in death saves.

Really cunning foes will hit either who they narratively hate the most or who they think is weakest.