This has come up in past posts and you can use the search function to find them. I'll add that the radii on the holes in the Stoney Point/LNL system gauges tend to make their measurements a little short of absolute. It really doesn't matter in this instance, though, as you are trying to get the difference between the two measurements. less the jump you want to the throat, to be what you read on your loaded rounds.
Redding's Instant Indicator tool will measure the ogive to shoulder distance directly and could be used with the gauge, then on your loaded rounds to set the bullet jump. I built a version of my own about three years before Redding came out with theirs. Fitted it with a cheap dial indicator, but it works fine. Mine won't do all the different measurements the Redding does, but mine has different caliber adapters.
