Today got a chance to try clocking the taper.
My idea was to use actual collets to set the angle - clearly an iffy idea because they are not solid, but worth a try before using something more complicated.

When preparing the spigot I made sure it's diameter was the top of the collet range - ie when using 9mm/8mm collet, the spigot was 9mm.

Also having clocked the largest collet, I than turned the spigot down and clocked smaller collet and checked whether the readings were the same for both:



I have managed to clock the taper to within +/-.0008 for both collets.
I don't know if this is good enough ... but I will try to go with it