Cip,
The easiest way to change your CG is take a stick of lead, cut it into the little 1/4oz pieces and stick one on the back and fly, stick another on and fly, so on and so forth. Eventually you will find what you need. Just remember to keep the fuel the same and above all, don't get the plane on the CG edge with a full tank cause about 3/4 of the way through your flight your not gonna be happy... Good Luck!