Alright, so your lead screw may have been cut down from a windage screw, but not done correctly. That may be all the problem, or it may not be.

The point about the setting up of the mounts is called collimation; that is the alignment or not, of the axis of the bore with the axis of the mounts and therefore of whatever scope is in the mounts.

The two axes are not supposed to be parallel in the vertical plane, only in the horizontal (windage).

In the vertical plane there must be an "angle of declination" built into the mounts so that the axes of the bore and scope meet and cross at a point determined by the trajectory of the bullet and the distance to the target.

If your 100yd zero is still on at say 400 or above, your collimation is probably fine and you just need a replacement lead screw. Someone else may know where you can get one.