I wasn't saying the game is based on RL. I'm saying use that principle. Guys lose their effectiveness long before they get to 0%. I believe that holds true in HBD as well.
If a guy's stamina is 90, he can probably comfortably through 120 pitches. After that, his performance is likely to decline significantly. It doesn't matter if he's at 50%, 25%, 10%, etc. after that 120th pitch...all you really need to know is that he's hit his wall. Which is why you've been killing Stankiewicz.
As I mentioned previously, go through and look at how many late inning runs he's giving up. If it's a lot - which it seems to be - his PC is clearly too high.