Keep in mind that the effective mass of the tonearm should include the weight of the headshell, the mounting screws, and of the cartridge body itself. Given that that is the case, I find your estimate of the effective mass of 7.5 g to be probably too low, and probably representing the mass only of the arm wand or perhaps the arm plus the standard headshell. Also, you need the vertical compliance of the cartridge not the lateral. All of that said, it seems you want to mate a high compliance cartridge with a rather lightweight tonearm, and that should work with no worries about the math.