OK Eamonn - thanks a lot, that's really helpful. Points taken and understood - they will give me plenty to discuss with TapCo.
(Actually, most kits have a 3 or 4 start thread I think, unless they are acme type in which case they tend to be double)
I'd still be interested in hearing from other makers about the usefulness of various diameters, so I can perhaps get some sort of consensus.