Bean:

I have never known any particular "Formula" for determining tip weight - too many variables.

My generality is that if I hold the fuselage and it rries to fall to the outboard side, but not so much that I can't hold it with the one hand, it's in the ball park.

The tip weight is not supposed to be the thing that keeps the plane from coming in on you. It is just to be a counterbalance to the weight of the wires on the inboard wing.

To keep the plane out the engine is set with a little out thrust sometimes, rudder offset is used on almost all c/l planes. Another is having the inboard wing a little bit bigger than the outboard, and if the plane has flaps then differential, with the inboard flap having more travel than the outboard tends all by itself to turn the plane out of the circle in any maneuver.

Hope this answers a question or two, if it raises any feel free to ask.

Bill.