The problem is that the percentage of probability you change by editing params for lucky powders only changes the extra probability they grant. The chars themselves grant extra probability for any plus with lucky sets and premiums..
The only chance you got is to make it to be legit to have a piece up to +12 and include into _memo_add procedure for example..a little line with a query like this: update _items set optlevel = 12 where optlevel between 13 and 15 . Then the players would be cleaning the extra plusses by their own activity.