It is my understanding is the effective mass for the SL--1200G tonearm is 12g with headshell included. In addition, these calculators that are available online are strictly approximations at best. It is always best to measure the tonearm/cartridge resonance using proven methods.
Definitely with stock Technics headshell, i agree
And it is much better to use actual Test Record to measure (and to see) resonance than any online calculator which can give you the idea, but not exactly precise measurements.