I would like to know the approximate position of the front seat
airbrake handle with wheel brake applied.

Andy,
The wheel brake on the G-103 is set so that it comes on just before the air
brakes are fully open. If you move the wheel brake starting point forward, you
won't be able to get full air brake deflection. I would stay with the factory
specs and adjust seating position to allow full use of all controls.
JJ Sinclair