15/16-16 bore size

I'm designing parts for my cnc router to accept Thompson ball nuts with
a 15/16-16 thread. It was obvious to use a 13/16 bore but upon
checking my tap won't start in that size hole. Boring to .862 appears
to allow the tap to start but I am wondering if there is a standard tap
hole size for 15/16-16??? Has anyone else tapped holes for ball nuts?
Thoughts????

Thanks,

Jack