Yes, make thethread and register for the Mandrel, and then turn the tapers and thread, in situ, to maximise concentricity.
The Internal taper is 16 degrees (8 degree offset for the Top Slide ). I finish mine by CAREFULLY lapping with some emery and oil, wrapped around a collet.
DON'T push too hard or it will grab!!!!!!!!!!
The thread for ER32 is 40mm x 1.5mm pitch.
Howard