Some later boxs have a a hole drilled behind the output shaft that you stick a (IIRC) 7mm drill into through a corresponding slot in the back of the drop arm.
If not then without the drag link connected go all the way to the left, turn 2 full turns to the right, this is the centre of the steering box. Set the wheel correctly at this point, then I adjust the track rod to get the correct toe in with the steering straight ahead before setting up the front drag link to get the steering wheel back in the centre.