Jelco Arm mass and cartridges

I was thinking about a Jelco SA 250 S arm, but noticed the effective mass at 20g seem high compared to others I have looked at. I believe the 750 is the same.

I understand the headshell to be about 12 g. If you buy a lighter headshell, do you subtract that difference in headshell weight from the effectice mass of the arm ?

I ask as it would seem many of the MM/MI cartridges I would try would be a miss match.

Any info on the cartridges and mass with the arm appreciated.

Yes, if you got a headshell that weighed 8 grams the mass would be reduced by four grams and so on. MM cartridge compliance figures have dropped considerably since the 70s and 80s when many were very high so more may work than you think.
The formula for effective mass of a tonarm is:

Total Dynamic Moment of Inertia divided by (Effective Length of Arm)^2

You will note that increasing a fixed weight at the pivot results in less of an increase of effective mass than the same weight at the headshell.