A 20 for DUR means every fifth day he can pitch to his max, which I set at 1.25*stamina. So with a 5-man rotation, I'd have this guy maxed at 80.
If he was a 25 DUR, I kick his max pitches up to 1.33*stamina in a 5-man rotation. I haven't had any fatigue issues at the ML level this way.