The math is easy if you use 100 grams per cubic inch for steel. If it was me I'd cut 3 cubic inches off the counterweights and then spin it on the balancer. 3 cubic inches is probably less than 0.100 of depth but you need to measure the length and width of your counterweights to be sure. Do you have a rigid setup on your lathe that will let you cut the counterweights without chatter? We tried some different setups over time and eventually hit on a carbide cutter and a feed and speed combination that produces a mirror finish with very little noise. But the setup has to be dialed in and rigid.