Reputable gage makers will insist that you buy a setting plug for any special rings you are buying because it is required to ensure the job is done right. Less than reputable makers will use the tooling they produced the ring with as a "master." And once they've used it on one job they may rework it for another.
When standard thread rings are involved, many calibration laboratories and gage makers keep setting plugs on hand. If special rings are involved, it is possible that the maker may have the right setting plug available; if the pitch diameters are weird, however, it is unlikely. This means that you will have to check with every gage maker you can find in case they have a plug. If they don't, you'll have to buy one to get the rings reset properly.
Resetting rings using readings off of a measuring machine will not do the job properly any more than using measurements off of the minor diameter. And don't even think about using a working gage as a master for your ring to be set to.
When special rings are involved, you're going to have to get a setting plug sooner or later so it is better to buy one when you get the rings. There are other reasons to keep setting plugs on hand for both your standard and special rings. Here are a few of them.
Years ago, when you sent your rings for resetting to a gage maker, they would lap them as part of the process to ensure the gage was "as good as new." Today, it is rarely if ever done this way, probably due to the costs involved.
The adjustable thread ring gage is unique to North America. Everyone else uses nonadjustable gages. There's no doubt about it from a metrology point of view-the solid ring is better. Part of this is that while an adjustable ring may be round when new, subsequent resetting without being lapped to fit means the ring will be out of round along with other problems that will affect its functional size.
But before the pro-adjustable ring folks get out the long knives, let me say that despite its problems, the adjustable thread ring-properly made or set-is more than adequate to do the job it was designed for. And it has the advantage of being cheaper to buy and cheaper to calibrate than its solid counterpart.