Originally Posted by
oneclick
1. measure the exact thickness of your jaws;
2. measure the end of the spindle - the very end;
3. measure the spindle with your jaws' outer faces flush with the spindle end;
4. do the math i.e.
width at 1.5mm = (width at end) + ((1.5/jaw thickness) * (width at jaw-flush - width at end))
There generally is no "very end" of the spindle to measure; the end is typically rounded over somewhat, and in any event, even if milled to a square edge, that's impossible to measure without cheating up the taper a little. Look at OP's photos, obviously there's a roundover or chamfer on the very end of the spindle.
The ISO approach is right: standard dimension at some distance from the end, but 1.5 mm is not a practicable number, IMO. The only way would be to measure the taper at whatever thickness your caliper jaws are, then "do the math" from that point to 1.5 mm from the end,
assuming that the taper is indeed 2 degrees.