+1, Raul.
In the calculation for effective mass, all mass counts, including the weight of the cartridge itself, the weight of the headshell, and the weight of any hardware. Thus using heavier than normal screws will increase the calculated effective mass, M. Because the screws are essentially in the same physical location as the cartridge, and because that net mass is at the very end of the tonearm, adding heavier screws has a more or less linear effect on the effective mass, M. In other words, if the new screws weigh 2g more than the old ones, add 2g to effective mass. Since resonant frequency is in the denominator of the equation used to calculate it, any increase in M will lower Fr.