counter weights only take out a longitudinal imbalance caused by the rotors being offset from each other. True dynamic balance of a single rotor would require the same weight as the rotor in opposition, at the same moment arm length. Anyone with a balance machine could spin up the assembly and tell you how much weight to add to the counter weights. Just put a rotor on the crank and the counter weights and spin it up. Plus, there is the oil weight trapped inside the rotor. So you might want to deduce how much weight that is.