Ok the workshop manual version for the RRC, I assume the 90 will be the same.
Disconnect the front drag link.
turn the steering fully to the left, you may want to put the steering wheel straight at thsi point.
turn 2 FULL TURNS to the right, the steering box is now centralised. Adjust the front drag link to bring the steering wheel back to centre when traveling a straight road with minimal camber.
On later models (300TDi IIRC) there is a 7mm hole slot in the back of the drop arm and a hole above it to centralise the box, but the effect is the same.
Any discrepancy between available left and righ lock is then down to the axle steering stops or perhaps a faulty steering damper or incorrect length damper, the steering damper on DI/RRC fouling the ARB or other obstruction.