If the machine is new, the gibs should be correctly adjusted from the factory.
If they are slack, you will never get repeatable, results If the machine is used, wear will be present which will make life more difficult. Ideally, the extreme ends will show least wear and can be used for setting.up.
Te objective is is to align the table so that the ends of the table surface are the same distance from the spindle,
ie the table surface is parallel to the plane in which it moves. In this way, the clock should read Zero at both ends of the travel…
Once you have established that the surface being used as a Datum is the same distance from the spindle at both ends of the travel, it should be locked in that position in the Y plane. Since the table is not being moved in the Y plane, (ideally locked there) it should not matter whether the column is dovetail or round. the distance is is constant; the only variable is the angle of the datum surface might be relative to the plane of movement in the X plane.
Having aligned the table, the T slots can be checked for parallelism to that datum surface.
If they are not, each surface should be skimmed, so that it just cleans up. The newly machined surface is the parallel to the datum.
You can then skim the other side of the T slot to clean it up in the same way.
When all the T slots have been skimmed to be parallel, then you can measure width, prior to opening up the narrow ones to the the width of the widest.
You should then have four T slots that are parallel to the datum surface, and parallel in width. It does not matter particularly if the slots are now 16.2 mm or so wide where they are a nominal 16 mm.
Within reason, the nuts do not need to be an air tight fit in the T slot. In fact, with the pillars being used as dowels, you do not use T nuts. My device is clamped by two discs, with two flats to make them a sliding fit in the bottom of the slot, and tapped to take the securing bolts..
You then turn the bottom end of the pillars to be a snug fit in the T slot, it is essential that they are, since they are the dowels locating the pillars, to which the crossbar is located by the V slots on it…
If they are not you will not get repeatable results.when you align the vice.
The accuracy of these operations determines the accuracy of any future operations.
Howard