So here's how I got my rule-of-thumb from above. Start with elbirdo's formula for how many pitches you get per pitcher-inning
3.406*BFIP + 3.762*BBIP + 1.964*KIP
Where BFIP (batters faced by pitcher) is WHIP+3, and BBIP is BB/9 divided by 9, KIP is K/9 divided by 9.
Let's take a typical deadball pitcher, with about 1.00 WHIP, 4 K/9 and 2 BB/9. (Compare 02 McGinity, who is used a lot in the 80M league, as an example of about this.) Such a pitcher would be allocated 3.406*4 + 3.762 * 2/9 + 1.964 * 4/9 = about 15.33 SIM pitches for every IP/162. So for every 162 SIM pitches we want thrown, we need 162/15.33 = 10.57 IP/162 from such pitchers.
So if everyone on the staff was like that, and we were cutting it super fine, we would want our staff IP/162 to be 10.57 times the number of pitches per game we expected to throw. But that wouldn't allow any slack for recent use fatigue, or getting through a game after a 16-inning slogfest, or anything else. So to be safe, I typically round up and work with about 11.
Though note how different the formula looks if we have live ball pitching. Take a good live ball pitcher, of the kind you might use in a $90M or so league. Let's say they have 1.00 WHIP, 9 K/9 and 2 BB/9. That pitcher gets 16.42 SIM pitches for every IP/162. If the staff is like that, then you only need 9.87 IP/162 for every pitch per game. And then the rule I suggested above, draft 11 IP/162 for every NP/G, seems very cautious.
Still, I think one of the biggest differences between the $80M and $90M leagues is that although people drafted similar numbers of IP/162 in the two leagues, they drafted very different number of total SIM pitches, because there were so many more strikeout pitchers being used in the $90M league. (Note that the average team has 2.5 times as many Ks in the 90M league as the 80M league.) And that's why there is so much less fatigue in the 90M league.
One thing I don't quite know though, is why the pitch counts are so low in the 90M league. It could be that we've just all been lucky, and fatigue is about to set in. But I think what's really happening is that the SIM doesn't add on as many pitches for a K as it 'should', given the formula it uses to allocate pitches to pitchers. If that's right, some of us (well, me at least) have been undervaluing high-K pitchers. That's a plausible conclusion; I almost never draft high K pitchers except in very high caps, but maybe I should have been.