Figuring Cartridge Compliance.. Help!

Dear @coachpoconnor :  I don't know which tonearm you have but the 103 is not really a to problematic cartridge .

You can use really " safe " a tonearm from 12grs. ( even a little lower. ) and up and will stay inside/around the normal resonance frequency range.
It's more important that your tonearm be a good damped design.

Regards and enjoy the MUSIC NOT DISTORTIONS,
R.

And if you really want to measure an exact cartridge/tonearm resonance practically (not theoretically) you need a Hi-Fi News Test LP 

Chakster... Thank you, I do plan to buy the test LP you suggest.
I bought a Harmon Kardon T65C in very good condition at the suggestion of an audiophile acquaintance. I really did not plan on putting the DL 103 on this turntable; but the other night I hooked it up to see how it would sound, out of curiosity. I have since listened to it for approximately 30 hrs. Last night I attached 2 gms to the head shell (and adjusted the counter weight accordingly) and it seemed to make a qualitative difference in sound. It got me interested in understanding how to calculate effective mass. I have seen some brass counterweights and brass head shell weights, that I also thought I would experiment with. But I am interested in learning to do the Math behind these theories.
rauliruegas- I believe that the effective mass of this Ito Microrace tonearm is 9.5.

That you chakster. I was getting really tired of saying that. 1+

Dear @coachpoconnor : Adding weigth at the headshell increment the tonearm effective mass and this is what you did it, good.

With 9.5grs-10.00grs. of tonearm effective mass you will out of the resonance frequency range around 1 hz.

Anyway here the Maths you want:

http://www.cartchunk.org/audiotopics/ToneArmMechanics.pdf

R.