The matching of cartridge compliance to tonearm effective mass is quite "plastic", in that there is a lot of room to make them match, if you use the standard goal of achieving a resonant frequency of 8 to 12Hz. Plus, in the modern era, we are inundated with "medium mass" tonearms and cartridges with manageable compliance numbers, so for the most part if you are using new OEM stuff, you needn't worry too much. MC, who discounts the importance, actually uses a tonearm that matches well with his cartridge, which is maybe why he can say he pays no attention to the issue. If you look at the equation for the resonant frequency (Fr), it depends inversely upon the square-root of the product of tonearm mass, M, multiplied by the cartridge compliance, C, at 10Hz, as Chak says. If you go on line and look up one of the calculators for Fr, you can fool around with the values for M and C and quickly see there is a lot of room for varying one vs the other parameter and still being in the OK range. This is because you are taking the square-root of the product of those two parameters, which mathematically jams the result into a narrower band of values that tend to produce an acceptable outcome.
By the way, "weight" as we commonly use the word, has little to do with the calculation. "Effective mass" is a product of the distribution of the mass of the tonearm, starting from the counter-weight to the pivot, and from the pivot to the headshell and cartridge, to include the mass of the cartridge and the mounting hardware. Don't knock yourself out with this stuff. Probably your Rega tonearm is in the broad category of "medium mass". Then it depends upon how much your cartridge adds to M and what is its C.
