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Yes, you're using the gauge correctly....
Re the TR25B... that's a different style of internal thread chaser I've never seen before; I'm used to the kind, like the earlier Sears/Snapon set, that look like thread cutting taps but aren't. All I know is the earlier ones in this discussion work fine so you would be safe purchasing them. And I would observe that the TR25B set will be fussier to use since you will have to dismount/remount the "blade" for each different thread, rather than just select the right one and use it. And that for substantially more money you are getting only internal inch thread chasers, no external, and no metric.