This isn't exactly a "quick" reply as I've only recently found this forum. Still, better late than never
I agree with Wayne 100% in that gauges don't "measure" - they inspect for right or wrong. By measuring you always know where you are with relation to the tolerance.
I have invented and patented (and sell) a method for (among other things) measuring thread pitch diameter and can in fact also measure internal threads from M6 - 1/4".
I have on two occasions supplied thread measurement inserts for measuring diameter D on an internal thread. It is true that the tolerance for this is the only diameter (major diameter on an internal thread) that doesn't have a specified tolerance and is only given as "min." This applies to both Unified Inch Screw Threads (UN) and Metric Screw Threads. Of course this doesn't mean that it can't or musn't be measured.
My personal opinion on making the "perfect" thread is to use a combination of solid thread plug and7or ring gauges and pitch diameter measurement. A good thread is in the middle of the pitch diameter tolerance. Measurement also reveals how much of the tolerance is being used and whether or not the process is being controlled. A gauge doesn't reveal this. Inspection by solid gauges usually requires a much larger sample size than measurement. Another advantage is that when setting a machine up, if the thread cutting tool is set for the middle of the tolerance process control can be reduced and focus moved to viual inpection of the cutting tool. Keeping away from the tolerance limits also means less wear on the solid gauge.
Another Rule of Thumb for UN and Metric threads is that, if the pitch diameter tolerance for an external screw thread = 1, then the pitch diameter tolerance for an internal thread is 1.3
The pitch diameter tolerance for threads is invariably the smallest tolerance on the three diameters. If the cutting tool isn't damaged or worn then measuring the pitch diameter is usually enough to ensure that the overall profile is OK and the thread within tolerance.
I have also made a simple observation with UN threads:
if tolerance 2A is known then all other tolerances (1A, 3A, !B, 2B and 3B) follow this one.
If anyone has questions then I'd be happy to answer - assuming I can