You may be able to find the manual on-line. If so, it will tell you how far back from the wing Leading Edge to measure your CG. You don't really need a fixture to do that. If low wing plane, put a line at the CG on top of the wing using a magic marker (you can use alcohol to remove it later) on both the left and right wing. Turn the plane over and put your finger on that line and see if it is sligtly nose heavy. If not, move the engine more forward and try again. If to nose heavy, move it back a smidge. If you have the engine as far forward as you can get it, then you will have to move the battery forward to help achieve that proper CG. If all else fails, nose weights (sticky lead weights available at your local hobby shop or at a tire shop (stick on wheel weights) will work, but less desirable (adding nose weight that is). You can also get away with a weighted prop spinner/nut. I believe Dubro makes one that is about an ounce or two in weight that works fine. If you go that way, make sure you get the right thread, by the way.

This also works with high-wing planes, just put the line on the bottom of the wing and do it that way. You should have a slightly heavy nose plane. You can adjust the CG as you wish later after you get some time on that plane. For instance, some 3D'ers want it slightly tail heavy.. but be very cautious. Tail heavy planes can be a handful. They take off just fine... but landing, well, that' often done by picking up the pieces. Slightly nose heavy will result in a smooth approach and landing as well as better overall handling while in the air.

Best of luck.

DS.