Tweak1 has nailed it. All the calculations are a waste of time. Even the bit about being compressed half way is just a guide or starting point. Another guide or starting point, the component should bounce very slowly, around 1 to 3 Hz. Just a guide. Only thing that really counts is the sound. There is no substitute for listening.
Most components have weight unevenly distributed. Springs don't have to be placed symmetrically. Don't have to be 4 per component, 3 might work better. Only one way to find out.
The main reason for all this trial and error tuning is Nobsound are not damped. Because of this they have a resonant frequency that changes with load. The same springs can give an extended top end, or big bottom end, or be tuned for a balanced sound. All just by changing the number of springs. Or adding weight. Mahgister does that, tunes his to within 1/4 pound.
Also the Nobsound design is more vertical isolation, with not much in other planes. That is the tradeoff for having terrific performance for cheap.
Townshend improves on all this, with damping that improves performance as well as flexibility. Height adjusts by turning the top of the Pod.
steakster, if you like piano on your system with springs, you should see the video where they put a piano on Pods!