I need a headshell recommendation for Technics 1200 GAE

If I’m reading the equation correctly, Z is an independent term that is simply added to the calculation after being divided by three. Since you’re only concerned with the delta here, calculating the change in effective mass is (I think) pretty straightforward. Whatever total Z is, the only part of it that changes with the headshell swap is the difference in mass of the shell plus screws, wires, etc (call it the shell package).

If the original headshell was (let’s say) 7.5 grams and the replacement package is 16.5 grams, you’ve increased effective mass by 3 (16.5 - 7.5 =9 and 9/3 = 3 grams) since that’s the unit of measure here.

The change in counterweight position and effective length should be very close to zero.  If you sub in a heavier counterweight and don't move the cartridge, it will be zero.

Yes?

Invictus - right you are, I missed the small m in the first term.  Since you can hold the distance of counterweight to pivot constant by increasing the mass of the counterweight (and nothing else changes), you would need to add the required incremental mass there to the sum I calculated.