Hi,
As a small but perhaps significant point I'd like to add to that I've noticed that player stamina has an influence. Last season I had 1-2 ST for my main trainees, but now when they're all at 3 ST, I normally don't get less than 47 minutes per game for them. It's difficult to get those 1-3 minutes as a reserve player, as in a blowout game the reserve might easily get 10 minutes and a tight match might keep him strictly on the bench.
Perhaps an ideal ST for training issues is at around 4-6 ST? This of course leads us to gameshape and other issues, but that's another story :)