I made up some simple kit to align my Rotary Table on the Mill/Drill in both Vertical and Horizontal modes.
With the Table set for Horizontal use, I used the equipment to align the Tailstock (very lightly clamped to the Mill table) with the Rotary Table, before tightening the locking bolts for height and Angle adjustments.
This aligned the Tailstock horizontally with the Rotary Table.Table at the same time. This used the the O D of the tailstock centre, which had already been set in a 4 Jaw Chuck on the lathe, to run true within 0.0005", and the Centre trimmed up . In fact the tipped tool merely polished the Centre, so I need not have worried.
At least I could now be reasonably confident that the Tailstock Centre would be well aligned with the axis of the Rotary table.
Howard