[TurkeyDriver]

This link should help explain how to find MAC. It is relatively easy to do but hard to explain in a forum text format.

http://www.airfieldmodels.com/informatio..._chord.htm

Also some basic physics will let you solve for adding gas (ie weight) to the aft tank and how it moves the Cg further aft.

Most 3D CAD type programs are able to solve this for you automatically.