440 crank

Here's my guess as to how it might be calculated:
A particular counterweight's thickness and radius is easily measurable. So is the weight's fraction of a complete circle around its circumference (number of degrees out of 360 degrees). Treat the counterweight as a solid cylinder with volume t * pi * r^2*. Density of 4340 steel is about 0.284 lb per cubic inch. Multiply it all and I'll bet it's close enough for minor drill adjustments since the amount removed is a small fraction of the counterweight diameter.
(An accurate calculation would be more complicated since you have an original radius and a new cutdown radius). More likely it is just common knowledge that you knock off .120 and go from there :rolleyes: