Seems rather bogus.

You can change equipment during practise. How would that be calculated? Equipment during start of practise? When finished?

Doing some basic testing now.

First result: The displayed values are incorrect. Naked Stat * Clothing bonus is not equal to the final displayed stat. Cindy for example starts with 105 in Tricks. Equipping a +25% does give her 130. It should give her 131.

Training her for 10 minutes gives 30 displayed at stamina 463. By your calculation it should give her 30*1.25=37.5. After training her she had "+39" displayed in the stat bar. Naked again the stat was 137. Changing to a 70% outfit it went to 228. If she had gained the "+39" displayed in the stat bar above, why are her naked stats not 105+39?!?!

Noooow. If she had an permanent +5% tricks bonus due to who she was...

100 with 25%+5% gives 130. Correct.
Training 30 gives 130, with 5% naked bonus she would have 136.5=137. Correct.
130 with 70%+5% gives 227.5=228. Correct.

From what I observed, all girls have 20% "girl" bonus. Cindy has 5% Tricks, 3% Commitment and should have 12%Sex appeal. My Cindy's Sex appeal was trained, so I had difficulty, recalculating, but i works out. She has "naked" 16.5k and equipping 70% just does not add up correctly. But if I divide 16.5k by 12% and then multiply by (70%+12%) I get the correct value of 26.8k which is displayed.

Everything you do affects the outcome of the training session! How did you not notice this?
Because it is not so. Your math is flawed by the inherent bonus any girl has.