This could work in either direction, but could make encounter building a fairly simple task again - it's just a thought.

The party's handicap starts a 1. This is what you multiply the encounter difficulty by when you're using the standard encounter calculator. So at the start, you are using it as normal.

After 2 or 3 encounters of "hard" difficulty, you notice that the players have wiped the floor with your monsters. So you put their handicap to 0.9. Now, each encounter you build has its difficulty multiplied by 0.9, and so they will get more difficult encounters. 

Alternatively, you can increase their handicap to make the encounters a little easier, so a handicap of 1.2 makes each encounter 20% higher on difficulty. 

This doesn't affect the actual fight, so you don't need to remember anything when you're fighting.

Has anyone done anything like this before, to account for particularly dangerous or noncombative parties? I'm going to trial it myself, but as my party's only had 1 encounter thus far I'm not changing anything yet!

Yeah the standard encounter calculator is pretty worthless for my situation. It depends a lot on your players level of optimization, their level of coordination, and how you set up encounters and play them out. I very much recommend adjusting things based on how your party performs in your sessions. Multiplying by an incremented handicap is a lot more exact than my 'toss in 3 more monsters, they can probably handle it' method, but it sounds like a solid approach.

Thus far my party has managed one encounter with ease, though it was in a large space so the ranged people could hang back whilst the melee ones charged in, so it let them play to their own strengths. I might make myself some notes on how the environment affects it as well...