The simplest way would just to remove all the radio gear from your plane and add weight to the nose so that roughly 20% of the area of the wing is in front of the C.G. If that is not convenient, you can make a small scaled down profile balsa glider of your plane and balance it in the same way. You may find that you can move the C.G. back as far as 25%, but 20% is a good, safe place to start.