That's why my level almost never leaves the truck on a as-is remodel job. To make things look and work right, they have to match what's there, even if it's not perfectly level or perfectly plumb. If later on someone wants to correct the original condition, then my work will follow it all back into it's proper place. Until then it looks as good as the rest does and it works fine- what more can you want? OCD (or CDO ) is good for us to a degree, but when wanting perfection blinds us to the real-world conditions we're working with it goes against us
The idea is not to do the job perfectly by your standards, but to give the customer what is going to be best for them without compromising quality. A perfectly level or plumb addition or renovation on a out-of-level or out-of-plumb house is going to look bad and it's not going to sell your next job; instead it's going to make you look bad no matter how perfect your work was. Go with what's there and you won't go wrong- if it's so bad that you can't do that, then you need to fix the other problem first or walk away from that job before it bites you in the butt.