If you do not have a scale, you can make one from a carbon tail boom support rod. Tie a string around the center to support it, then measure out one side at 5" and tie another string to it for hanging the piston-pin-con rod assembly. Tie another string on the other side at 10" for the weight. make a weight to ballance. The weight is nothing more than a cylinder of metal with a hole in it the size of the crank pin and about 1" in diameter. Make the thickness to suit the needed weight.

Do this again for the con rod alone and use a ratio of 2-1/2" and 10" to get the 1/4 weight ratio. Add this weight to the first weight and ballance the crank.

Crude but effective if you don't have a scale.

Wolfgang