Accuracy is everything, I've found. I used the self centring lever of the T/X so that at least one parameter was constant. I fine tune by using the extra centring lever adjuster and try to get it to not move left or right within two clicks each side of centre.
However ....
Upon testing today I've found a flaw. When you switch on the set up, the servo judders a fraction and starts revolving minutely until you joggle the direction control.
Now this might be OK unless you forget at the lake and find it's started to turn unnoticed and and moves something it shouldn't like a crane pulley. !!
Funny you should mention leaving the pot in the circuit Malcolm, as I played around with this idea and found it worked as it should but it was ultra sensitive compared against the fixed resistor idea. Any vibrations might alter it's position.
I was trying to save costs of 3 pico controllers but obviously, this is the way to go. I assume they reverse, although it doesn't say so.
Also in the above description I forgot to mention that one of the gear wheels has a stopper block built in and THIS has to filed off to allow the wheel to travel 360 degrees.
Overall I'm not happy about using these on my crane but I'll give it a try to see how it works out.
ken