You are absolutely right in that the line pillars have to be exactly the same length, otherwise you will get a run-out on the face.
I made a lot of mistakes doing this, but found that the best way was to firstly cut them all as accurately as you can to the desired length, allowing about 0.005" (about 0.1mm) extra. You then measure them all, put the shortest to one side then carefully cut a tiny bit of each, one at a time, until they are all the same as that shortest one. It also might help to make an extra one-or two, just in case one goes undersize..and this does happen.
I also made a "gap gauge" about 0.001" shorter from a piece of spare metal and when you assemble the two plates with the pillars, put the gauge between the plates where the pillars are, each pillar in turn and do up the screws until the gauge just touches. By the way, you minimise distortion if you do the pillar screws up in opposite pairs-rather like the nuts on an engine cylinder head are tightened.
Hoping this may be of some help..
wm+
