If your referring to relief pitchers, I tend to run mine about (setup guys) about 5 to 10 pitches below stamina depending on durability. For example a 75 / 35 durability guy I would set at a max of 30 pitches, with a target around 20. I've found that they rarely fatigue. For long relief pitchers I do use the durability more as a guide, if a guy has a 50 stamina, but only a 25 durability I would cap him at about 30 pitches with an target of 25, he'll miss a day here and there but can give you an inning or two. If his durability was in the 40's or higher, I'd likely top him out at 40 max. For relievers I never exceed the stamina number with a max pitch count.
For starters the thing I've noticed, and tec or mike correct me if i'm wrong but all X pitch counts for starters aren't created equal. Say a pitcher throws a nice 110 pitch 8 inning start only gives up a run a couple hits and a walk or two, in my experience his stamina will drop to the 40 to 50% range and then start the recovery process, however if that same pitcher throws the same number of pitches but doesn't get past the 5th inning and gives up several hits / runs, then his stamina is markedly lower even with the same pitch count.