I'll take a stab. This is for HS with the thought that a coaching staff can reasonably keep track during the course of a game/week...
14-15 y.o. - 1st month of season max = 75, thereafter max = 84. No new hitter when max is reached. 30+ pitch inning = -5
16 y.o. + - 1st month max = 85, thereafter max = 94. 30+ pitch inning = -5.
1-30 pitches = 2 day rest
31-50 = 3 day rest
51-70 = 4 day rest
71+ = 5 day rest
This would protect as well as ensure that pitching depth is developed.