In the few F3A planes I've had the vertical CG has tended lie within the wing itself so inverted or upright the "pendulum effect" has worked against me.

Using a set of scales to determine the CG via maths works well and for those with a bolt on wing and suitably stronge fillets, so does a 4mm wide cable tie slipped between the wing and plane with the wing bolted loosely in place. Just slide the tie along and keep lifting the plane until the wing rests level.

There's lots of ways to skin the cat, but as has been previously stated it's only a ballpark figure as you'll fine tune it later to what you feel the plane needs.

Another handy tip is if you want to move the CG a set amount, multiply the weight of the plane by the distance you want to move the CG (eg 10lbx1/4") , then divide that number by the distance from the CG to where you can put the weight ( down the back 50 inches ? ), and that will give you how much weight you need to add... (10lbx1/4")/50" = 0.05lb or 0.8oz

Of course you'll reposition your gear later so as to remove the dead weight