Okay, thanks for that info. As far as the pivot point goes, there seems to be quite a range among commercial machines, and even a wider one among homebrew ones. Mechanical engineers will argue this physics, but all these recorders seem to work just fine. I was looking for some magic 'golden mean,' but perhaps it's not as terribly important as the purists make out.
I thought I had posted the Keywood paper here somewhere, but can't find it. If you need it, it's here: https://www.dropbox.com/s/140rmbnaulsl2xa/Vertical%20Tracking%20Angle.pdf?dl=0
An old Fairchild lathe I had years ago used only a counterweight, and I recall the same problem maintaining a constant depth of cut because of the vacuum hose and wires. I think I clamped those to the carriage near the pivot point, so at least the head was no longer sensitive to changes in the dressing of the longer run to the chip jar and amplifier. A spring allows adjustment on-the-fly, of course, but it does raise the resonant frequency of the vertical component, and if the 'Q' of the system is high, can lead to nasty vertical oscillation. I guess part of the trick is keeping the length of the spring as long as you can so that the stylus pressure doesn't change much as the vertical position of the head changes. I don't know about these mechanical analogies of equivalent electrical circuits.
But the bottom line is, of course, that you are getting splendid results. Good job and good luck!