hmm. Here's how i do it:
Plug your figures into this calculator:
Is this a flat wing, or an airfoiled plane? It does make a difference in the cg.
I wouldn't go any further than 17% mac, i usually start at 12-13%, and move it back, if necessary.
13% mac, is about 14 1/8" behind the lleading edge at the root, and 17% mac is 15 3/8" behind the leading edge, at the root.