Faced with the same problem, this was how I went about it.
Remove centre from tailstock, mount in lathe and trim it, (Mine was hard and OK! )
My tailstock had Dowel holels in the base, so turn up two stepped dowels and fit to tailstock.
Fit an ER Collet chuck to Rotary table.
Put a collet into the Collet Chuck, that fits the OD of the Tailstock centre.
Clamp Tailstock to Milling machine table, leaving adjusters loose.
Load Rotary table to Milling machine table, but leave fasteners loose..
Grip Tailstock centre in collet.
Clamp Rotary Table to Milling machine table
Tighten clamps fo0r the tailstock centre.
The Tailstock centre is now correctly aligned in the vertical plane.
And the Rotary table is aligned with the Tailstock.
It is worth positioning the Rotary Table at one end of the Milling machine table, so if there is a need to reposition the Tailstock, the dowels will maintain alignment in the horizontal plane.
HTH
Howard