Hi Mark,
I needed to regrind the taper on my Boxford 1130 which has a D1-3 camlock nose.
After much thought I came up with this gauge to get it right.

The gauge is a lump of steel with a plain hole bored to about the mean diameter of the taper of the nose. There are three micrometer heads at 120 degrees which read zero when the spindle faces are level with surface of the gauge. I measured the bore diameter accurately and then calculated how much the three micrometer spindles should project such that when the bore was pushed on the taper the faces of the spindles would touch the flat surface of the nose.
I then tried it on a CVA toolroom lathe with the same taper and, much to my surprise, it worked perfectly and the calculated projections were perfect.
On my own lathe I set the topslide to the taper with a 1 micron dial gauge and cleaned up the taper. (The taper was really OK but the flat face had damage and you have to remove a tiny amount from the taper which equates to quite a bit off the flat surface.) I then ground just enough to clean up the flat surface with a flared cup wheel and tried the gauge and adjusted the micrometers until all three gave the same reading whilst the bore was in contact with the taper. (This takes quite a bit of trial and error but doesn't take that long.) This told me how much more to take off the flat surface.
It's certainly good enough for my purposes now.
Hope this makes sense.
Cheers.
Phil.