I think you need to access the forward/reverse control potentiometer
This should get you to were you need to be.
1. Turn on the truck with the tiller in the rest position and with both the lift and lower buttons on the left hand side held down together, release the buttons after 5 seconds. The tester screen is displayed on the DDI
The screen shows the control inputs COMBI INPUTS, press the left hand lower button for 5 seconds and the tiller head calibration screen will be displayed on the DDI, SCREEN (Tiller Analoge input.

2. Moving the butterfly forward slowly to full throttle the curser moves left on the input screen, then return to the centre, move the butterfly slowly in reverse to end of stroke and return to centre, do this three times.

3. Press the left lift and lower buttons at the sme time to save the calibration and release after 5 seconds. Switch of the switch. Switch on again and see if this has cured your problem, if it dose not sort your problem you may need a new tiller board.

Regards Nero