im starting an Eberron campaign. Six level one characters. Was thinking of the opening encounter being against Iron Shadow hobgoblin. Wanted the encounter to be hard but not deadly. I’ve been considering two ways of playing it. Either one Iron Shadow going all out fighting to the death. Or two of them, w/o charm person, who only fight to half health. 

I think your second option is probably a solid hard challenge. That 4x multiattack could quickly kill a PC though. Maybe use two, but they only get 2 or 3 attacks.

A useful tool for getting some sort of baseline for an encounter's difficulty would be in the 5e Dungeon Master's Guide, Chapter 3: Creating Adventures, Creating Encounters section, Creating Combat Encounters topic. Its a good bit of math that can totally be inaccurate once you factor in party builds and player resourcefulness and gear that is awarded differing from what is considered "normal" for D&D, but it can give you a number from which to start working forward or backward as needed.

The types of encounter difficulties listed are Easy, Medium, Hard, and Deadly. I'll add a very paraphrased description of what each is considered.

Easy - Doesn't tax character resources, might lose a few hit points.

Medium - No casualties, a healing resource or two might need to be used.

Hard - Weaker characters might be taken out of the fight, small chance of someone dying.

Deadly - Requires good tactics, defeat possible, one or more characters could die.

A DM I've seen typically throws encounters that are beyond Deadly at a well-rested and fresh party cause the characters carry extra resources and all have pretty good magic items.

Using your scenario as an example, I'll go over the math for it.

Hobgoblin Iron Shadow = CR 2 (450 XP)

Using the chart in the section I listed, it suggests that each level 1 character can handle 25 xp worth of monster for an Easy encounter, 50 for Medium, 75 for Hard, and 100 for Deadly.

So six level 1 characters could handle 600 XP worth of "monster" but it would be a Deadly encounter that could make someone die. Since a single Hobgoblin Iron Shadow, fighting to the death with all its abilities, is 450 XP worth of creature, it would be listed at the exact amount for a Hard encounter for six level 1's (75 x 6 = 450)

When you add more creatures in there, there is another table that lists how much to calculate the change (since it creates a bigger HP pool and more attacks.)

The multiplier for 2 creatures is 1.5. So (450 + 450) * 1.5 = 1350. So two Hobgoblin Iron Shadows would be beyond the Deadly encounter range for six level 1 characters. But you are also changing it so that they fight for only half health and don't have their Charm Person spell. The rulebooks don't really cover every single possible change with math, of course.

The Dungeon Master's Guide also warns against putting a party against a creature whose CR is higher than the party's average level as that sometimes makes the abilities of the monster far outweigh the resources of the party. It specifically uses the Ogre (CR 2) as an example saying that it can easily one-shot a level 1.

Anyways. This probably didn't give a straight answer, but hopefully it pointed you in the right direction as to what to measure.

(Oops, had to edit this quickly as I forgot to add the XP value of the two Hobgoblins together before multiplying it.)

I did use the DMG. Just after the adjustment for number of monsters it goes over readjusting those levels for parties of six or more. 

Party Size

The preceding guidelines assume that you have a party consisting of three to five adventurers.

If the party contains fewer than three characters, apply the next highest multiplier on the Encounter Multipliers table. For example, apply a multiplier of 1.5 when the characters fight a single monster, and a multiplier of 5 for groups of fifteen or more monsters.

If the party contains six or more characters, use the next lowest multiplier on the table. Use a multiplier of 0.5 for a single monster.

Given that I thought a single might end up being to easy. (Even a CR 2 would have a hard time sustaining 6 1st level attacks.) So I thought throwing two at them with some small tweaks would be more what I was going for. I think the suggestion of reducing their multi attacks is good though. And if they happen to take both out then they get some bonus XP. 

Ah, I didn't read that part as I was researching the encounter builder. I just remembered that the encounter builder was in there somewhere. Thank you.