Once the head is trammed,parallel to the table in X and Y planes, the problem is is maintaining position after the head is moved up the column.
Stan Bray advised having a cheap laser mon the head, aimed at a line on the furthest wall.
With a cheapie laser line. from a DIY shop, aimed at a pencil line ten feet away, it keeps things within a thou or so.
For greater accuracy, you need to be consistent whether you use one side or the other, of the centre, of the pencil line. the further the line is from the machine, the more accurate you can be.
DON'T ever look at the laser if you value your eyesight!
Howard
One day I'll proof read before SEND!
Edited By Howard Lewis on 17/07/2020 17:23:31