Thoes are programmed things, if your player has higer phy&morale skills (expecially Fitness) he won't lose so much.
I usually try to get atleast 2-3 point's higer Fitness than player's quality points if im going to train him.
Let's say that im training an ST, i would keep the Attack & Phy/moral skills 3points above and lower the defence by 6 or so..often my players get lower than 3-6-9% on thoes trainings
BESIDE THOSE FACTS.. NORDEUS SAYS ITS AVERAGE