If it's a built up wing and you can tell where the spar is, you can balance it on the middle of the spar and be right 99% of the time. Also anywhere from 25% to 33% of the MAC (mean aerodynamic cord) would be right. I'd start at the 25% point and fly it there. Then if you want you can move the battery around and progress back, in small amounts (1/8" or so) till it flys like what you want.