What I think would work:
If the "original" is 2 inches and I want it to be 6 inches, I would use the 300% setting.
Basically, divide the size wanted by the size of the original.
6 (wanted) / 2 (original) = 3 = 300%
2 (wanted) / 6 (original) = 1/3 = 0.3333 = 33%