Bluedeath,
Before define degrees for joystick travel measure how many degrees
you get with you HALL assembly, and use this to define pole length,
base hight.
For example, if you use a 75 cm pole - approximated height need for base
in floor with conventional desk chair - to achieve + 20 - 20 degrees
in HALL, you need around 52 cm of travel in top of stick, that is too much,
causing conflict with legs.

I suggest to you make some simulation with some CAD soft - the freeware Google
Sketchup do this and is easy to use.
Sokol1