It also assumes that the machine shop has a good hand that knows what he is doing, rails go face down on machine(mill/shaper/grinder) operative packs any gaps to the table making sure not to distort the rails then machines the pads, rotate the job pads down (now clamping on pads) on machine base and machine the top and at least one side.
Was thinking a decent size (thickness) shim so it could be setup in a 4Jaw chuck and machined on a lathe.
Wouldnt have thought it would be possible to get your alignment dialed in before your putty went off.
DIY Laser levelling using webcam
Regards
Mike