A x B divided by C = D
A is distance in cm from pivot to focuser centreline
B is extra weight of eyepiece
C is distance in cm from pivot to where you want to attach the counterweight
( in the opposite direction )
D is weight of counterweight
P.S. Assuming that your scope already balances with at least one of your eyepieces, then you need to work on the difference of the weight of that eyepiece and the heavier ones...... but realistically a scope should not be that sensitive to changes unless you're going from a small plossl to a N31T5
