There are several ways to estimate tonearm effective mass, but none of them are really simple to do. Safe to say that the static weight of the headshell is a factor, but not simply by the ratio of its static weight to that of the stock FR64S headshell. Best way to approach this, in my opinion, is to try it. You might also acquire one of the test LPs that permit estimating resonant frequency. From that information, you can calculate effective mass of the assembly of tonearm, cartridge, headshell, screws, and the counter-weight at the rear.
Another trick: If you think of the tonearm as a seesaw, with the counter-weight on one side and everything else on the other, then you can get at effective mass by balancing the tonearm such that the arm is hanging level in space, zero VTF. Now measure the distance from the pivot to the center of mass of the CW, in centimeters. Then remove the CW and weigh it separately, in grams. The effective mass will be related to the product of the square of the distance from the pivot point to the center of mass of the CW times the weight of the CW. Now, if I can only recall HOW the two are related, you'd be in business.
