To my way of thinking, 21 grams is in the range of "high mass", not "medium mass". Medium mass is more like 10 to 15 grams, IMO. Of course, to some degree it is a matter of opinion. In any case, 21 grams is not stratospherically high. Most notably, the well-liked (by some) Fidelity Research FR64 and 66 are much higher in mass. And some have mated the FR tonearms with relatively high compliance cartridges and report good results. You can easily find headshells that weigh as little as 6-8 grams, so as to effect a reduction in effective mass for the Jelco. But like Raul says, it is best to keep an open mind; don't be a slave to those formulae for mating compliance to effective mass.