My local FBO/shop
says finding neutral is not simply the 1/2 point between full up and
full down. You need protractors, string, and an advanced degree in
Euclidean geometry.

And an answer like that would turn me off totally to that shop. True, it
is not 1/2 way between the stops, but it also IS a simple procedure.
Given the slop in the plastic cover and the variability of the pointer's
position (when bent), why obsess? Fly it in still air and trim it off
level. Mark that position and you are done. There is no need to go
through complex and expensive procedures. Yes, it will change a bit with
varying loads, power settings, altitude, and temperature. The neutral
position is only approximate. So, approximate it!

Good Luck,
Mike