This won't be a popular answer, but I've literally given up on bittering units calculations. I go with hop-schedules that have worked well in similar gravity beers, and to hell with the numbers.
In a pinch, I'll use tinseth for dialling in, but that's just because it's the one I always used before.
If you're a stovetop guy, you get a massive difference in actual IBU from the size of the boil (bigger is better), the degree of the boil (I now leave my lid partially on to keep the boil violent, makes a huge difference), and how free the hops are (if you must use a hop-bag, make it a big one).
Cue comment from Kyle re: Quality Control