pure mathematics ... since a player quality is given by the average between 'defense 'attack and 'mental you should figure yourself that when a player gets 15 skill point it will rise 1 quality !

for ex a player quality 34 has to get the amount of defence + attack + mental equal with 102 ... if will have 101.8 means it`s quality is 33 and will need 1 skill point to rise up at 34..

i hope you will understand me !