To increase potential of a crafted item you must put stat points for its category Str:sword Dex:bow Int:staff/device Agi:knucle Vit:armor
Note:these are just examples.
The ammount and "zize" of a stat depends on your character lvl. +1,+1% stats and 1 stat slot every 10 character levels plus these require a certain number of material points (beast,wood,metal,mana & medicine) wich increase on a stat basis.
Not sure since its normally a random amount , by personal test an armor with potential 25-30 went to 27-30ish after 20 vit points where added sooo.. cant confirm it but maybe 10% for 10 points?? Still random ammounts and been getting 25-30+ ammounts