Luck is increase 0.01 for each 2 points in any of class. Hardcore, charm or Know-how.
whats happens is if you choose one item with more points at some class the luck will be higher i.e Exchanged Greaves + 30 HD for Magical Skirt +62 Know-how . Give more luck points after that because 62 is more then 30. you need to replace the item for the same local to test. like head for other head. armor for other armor etc.