without a lot of explanation:
using draft center stats
given Batters Faced per Inning Pitched (BFIP) = whip+3.00
given Base On Balls per Inning Pitched (BBIP) = BB9/9 or BB/IP
given Strike Outs per Inning Pitched (KIP) = K9/9 or K/IP
the number of pitches per inning that sim allocates for any pitcher can be found by
3.406*BFIP + 3.762*BBIP + 1.964*KIP
multiply by IP/162 to get the pitchers seasonal pitchcount allocation
ex. 03 Gagne
3.406*3.69 + 3.762*0.24 + 1.964*1.66 = 16.75 P/IP or 1390 P/Season
ex. '18 Northrop
3.406*3.73 + 3.762*0.08 + 1.964*0.10 = 13.78 P/IP or 730 P/Season
seems to be fairly accurate
includes 10% bonus that Sim gives pitchers over real life numbers
It should be noted that the calculations are NOT reflective of what the particluar pitcher may have pitched in real life. I know that sim assigns each pitcher has a predetermined seasonal pitch count. Earlier in the season or if you let a pitcher get overly fatigued, you'll get a mouseover message that says something like "Overused -- on pace for 3% more pitches than actual season." If you take his current pitchcount and extrapolate it to 162 games, you can figure out what sim thinks his seasonal pitch count is. Current ex. Gagne after 122 is 3% over with a pitch count of 1084. 162/122*1084 = 1440. 1440/1.03 = 1397 pitches per season. I believe my formula says 1390.
I assumed that sim must be using some sort of calculation based on the relative occurrence of three outcomes - 1. batter puts ball in play, 2. batter strikes out, 3 batter walks. So I took real reverse-engineered data for a bunch of pitchers based on the 'mouseover' stuff and did some curve fitting to come up with the three coefficients. The 10% is included because sim includes it in the mouseover message.
I gather Gagne real-life 110% number is closer to 1308, so I dont think sim uses real life pitch counts, but rather a calculation which I have attempted to duplicate.