I spoke with Frank Mari of Mari tools today. It is great when you can talk with the owner of the company and Frank has been very accessible when I have had tool questions.
Basically Frank said what a number of you have relayed - that single points due to their sharp 60 degree ends don't cut the same way taps do and need to be experimented with to find the right major diameter.
There apparently is no other formula than to just do some test cuts. If one needs tolerance then the use of GO/NO GO gauges will help guide.
Also, new to me, these thread mills can be used for metric as long as the TPI is equivalent and one just uses the same drill-size as with a regular tapping procedure.
I have dialed in the major diameters for the threads I am currently using and it is great to be able to teak them for tighter or looser fit.