In my posting yesterday, I forgot to say that my tests show that film-speed appears to drop a little when the ascorbicAcid/Phenidone ratio goes much over 100. Here's the curve I got when the ratio was 117:
Notice that the left 2/3 of the graph is slightly thinner than XTOL, but the right 1/3 is slightly denser. This means the neg has lower speed but higher contrast than XTOL (and same grain). The differences are small, though. Below a ratio of 90, activity begins dropping noticeably. So 90-100 seems to be the ideal range. It appears that the designers of XTOL made the ratio as large as possible so it could withstand as much ascorbate-loss as possible, but not so large as to lose speed.
I've also noticed that reducing the ratio appears to boost speed. The same seems to be true of reducing sodium sulfite. I say "appears to" and "seems" because I need to run more tests to verify these statements. Crawley's FX 55 developer was designed to achieve maximum speed, and claims a 1/2 to one stop speed-boost. Its equivalent ascorbicAcid/Phenidone ratio is only 11.6, and it has only 3.7 grams of sulfite, which seem to support my observations that low ratios of these chemicals give higher speed. Once I'm done with this XTOL-concentrate project, I'd like to experiment with speed-boosting developers.