How Can I figure out my Effective Tone Arm Mass??

Mount a cartridge and play the resonance test tracks on the HFN test record. This will give you the horizontal and vertical resonance frequencies of that particular cartridge/arm combination. If you know the mass and compliance of the cartridge, you can calculate the effective horizontal and vertical mass of the arm. (They're often different.)

I'm too rusty on the math to remember the formula, but someone here or on VA will certainly know it.

Effective mass calculations are not only considered in the initial tonearm design criteria, but also in the weight of the raw tonearm components. Also, effective mass is established in the static and axial weighting of all components comprising the tonearm assembly. Tonearm mass and cartridge compliance go hand in hand.

What tonearm are we discussing?

A Systemdek Profile tonearm on a Systemdek llX TT. It will go eventually but right now I want to get the TT going as best I can. I have 2 cartridges; A Grado Sonata and a Denon DL103. The first is light and the Denon is heavier. So I am trying to figure out which one would seem to be a better match.

You can use the method Dougdeacon described and look up the chart at the bottom of the FAQ posted on the VDH web site. This FAQ also contains over 200 Q&A covering almost everything related to vinyl playback, some of them are quite insightful.

http://www.vandenhul.com/artpap/phono_faq.htm

Great info, thanks!