Hi. I have a Myford/Rodney mill mounted on a very nice base. This is always a problem with rack and pinion z axis. My Myford drives the rack thought a worm and wheel which is hinged so it can be diss engage. I tighten up the vertical and always approach in from the same direction (very normal on a lead screws). The hand wheels marked in thou, I reckon about 5 accuracy but its a challenge. But it drives the rack via a worm to gear everything down.

Tom