The Grace 940 is an S-shaped tonearm with an effective length of 237 mm according to the specifications. If you look at the effective mass of several similar tonearms from the era (SME 3009 II, Micro-Seiki MA-505, Fidelity Research FR-64fX) they all run about 4-6 grams effective mass NOT counting the headshell. This is based on tests by Hi Fi Choice back in the 1970s and 80s. So figure that 5 gms is a good guess and close enough for the 940.
To get the total effective mass you then add the mass of the headshell (anywhere between 6 gm for the skeletal shells such as the perforated SME, to over 20 gm for the heavyweights) plus the mass of the cartridge. For example if your headshell weighs 8 grams and the cartridge 9 grams then the total effective mass is 5+8+9 = 22 gms. Note that the total effective mass is dominated by the headshell and cartridge weight, so any error in the weight of the tonearm minus the headshell is negligible.
The suitable cartridge weight range does NOT include the weight of the headshell. However, that range is likely based on the stock headshell, so if you are using a headshell that is significantly heavier, for example, you may not be able to balance a heavier cartridge even with the balance weight all the way back.
To get the total effective mass you then add the mass of the headshell (anywhere between 6 gm for the skeletal shells such as the perforated SME, to over 20 gm for the heavyweights) plus the mass of the cartridge. For example if your headshell weighs 8 grams and the cartridge 9 grams then the total effective mass is 5+8+9 = 22 gms. Note that the total effective mass is dominated by the headshell and cartridge weight, so any error in the weight of the tonearm minus the headshell is negligible.
The suitable cartridge weight range does NOT include the weight of the headshell. However, that range is likely based on the stock headshell, so if you are using a headshell that is significantly heavier, for example, you may not be able to balance a heavier cartridge even with the balance weight all the way back.