It should be OK. Note, the screw does NOT come out of the cover. It is captured in the top of the sector shaft. The screw threads through the cover and presses the sector shaft down. The further down, the tighter the mesh of the sector shaft / piston teeth.

If you unscrew it, it'll come out for a ways (the steering will get *real* sloppy as the sector raises with it) until the shaft bottoms out against the cover. At that point it should stop turning and you might be able to break the nut free. If you break the screw though, you're... well..., screwed.

To adjust the box once things are freed up, you need to make sure the steering wheel is centered. This is the tightest position. If you turn the wheels, then adjust, it won't steer back through the center position any more. You kinda tighten a little, steer back & forth over the center position, tighten a little more, etc. Stop when you feel it drag going through the center position, back it off a little and tighten the nut.

--Dan