A65l, I disagree. If one devides any number by 1/3 it is the same as mutipling by 3! To devide "invert and multiply". However your first statement is correct that more or less 1/3 of the cord is a good place to start if the main spar of the wing is at or about 1/3 of the weight and ballance, fore to aft. On "most" model airframes the center of gravity (main spar location" will be the same.

I hope this helps and please excuse the spelling but I am from TEXAS.

fly the airplane first

73 Lee K5MRC