The balance point on planes is usually 25% of the wing cord. Or so i have been told. Im not 100% on that but....you can try it.
Measure the wing, take that measurement and multiply it buy .2 That should be your balance point. I think that is just a rule of thumb and nothing concrete. But it seems to me with all the plane i have that the balance point is recomemened to be somewhere within that range. then take the battery, radio eqiupment etc. and move it back or forward to adjust balance. If that is not enough then add coins or sticky weights where needed. To check the balance point on the plane mark the balance point an lift it up and let it rest on your index finger. If it falls nose first then its nose heavey and so on.....