Just do it.
Keep in mind that any published figure for tonearm effective mass must be accompanied by some other information, if we are to make sense of it. We need to know whether the published figure includes a "typical" cartridge and the mass of the OEM headshell. As soon as you use a different aftermarket headshell, the EM would be altered. If your cartridge is unusually heavy or light, that too might change the result. Every single cartridge is not going to have a compliance exactly as stated by the maker of that particular cartridge; there can be significant sample to sample variation. Finally, the counter-weight has a variable effect on EM; the CW adds to EM in relation to its weight times the square of the distance between its center of mass and the pivot. So to minimize EM, you want the heaviest CW that works to counterbalance, so it can be as close as possible to the pivot. As someone else pointed out, the actual leeway for arriving at a calculated resonant frequency that falls within the acceptable range of 7 to 12Hz is quite wide, referring to the (probably inaccurate) numbers that you finally decide to plug in to the equation. Which is why I say, "Just do it". Unless there is a huge and obvious disconnect between compliance and EM, you’ll be OK.
