[Dave in MN]

I can't see why you would need to remove the arm. Not having read the procedure, Is it possible they mean to remove the pitman (drag) link from the arm at the ball joint? This would seem to make sense as it would be much easier to feel the movement without the drag of the rest of the steering assembly.

Every video I have watched shows adjustment with the arm still attached to the box. Center the steering wheel half way between extremes (lock to lock) and then in small increments move the pitman shaft adjusting screw in after loosening the lock or stop nut. Snug the lock/stop nut each time you test as just the play in the screw threads will make a difference in how it feels. At the point you feel any resistance in the steering wheel in the centered location, you are too tight, back off just a little.

I just adjusted two F100 boxes in the last two weeks. Great results with a bit a patience.
Good Day!