Since you did it 10 times, that is probably a good number. Problem is that the bullet might pull out a bit when you unload.
If it is easy to remove the extractor (like on a Savage) take it off and then just push the round out with a cleaning rod.
When I start making a new load, I just load the bullet long and chamber it. Then seat the bullet a bit deeper and repeat until the marks stop.
Then I use the tool to measure where the ogive is and that number gets used for future loads and different bullets. But this is why your tool needs to be a bit bigger, so it measure more where the ogive shoulder is.
The first time I went looking for the lands in a rifle barrel, I built something similar to the Hornady OAL tool. Complete with a case fired in that rifle. A lot of machine work, time, and trial and error.
The next time around (different calibre) I did what Ganderite just described, seating a bullet long in a dummy round and gradually going deeper until I had no more rifling marks in the Sharpie coating on the bullet.
With the bolt out of the rifle entirely, I simply pushed an empty resized case (no bullet) firmly into the chamber with a fingertip to get a feel for what the shoulder bottoming out in the chamber felt like.
Then I added a long-seated Sharpie-coloured bullet in the case, again pressing it into the chamber by hand. Very different feel, with no slap at the end since the case doesn't touch the shoulder of the chamber. I needed a cleaning rod to get the case out, the bullet was slightly stuck in the lands, evidenced by the equally spaced marks on it.
Then I seated 0.005" deeper and tried again. Repeated until there were no more marks on the bullet, and I was back to the same feel I had with the empty case alone. Very firm stop at the end, almost a slap, rather than a slightly yielding sensation as the lands start to bite into the bullet.
I don't know what it would feel like on a well- used rifle with a worn throat, this rifle was brand new, never fired.
I concluded I was within 0.005 of the lands, and tried closing the bolt as a final test on a fresh dummy round seated to the same depth. No marks were evident on the bullet.
Since I prefer "jump" to "jam", I adjusted my seating die 0.015" shorter to get between 0.015"-0.020" of "jump" and called it a day. 900 rounds later, I've had no problems and excellent accuracy.
I'm not suggesting anyone try this, just telling a story. If anyone sees a problem with my approach to finding the lands, I'm all ears. I'm old, but not too old to learn.
The Sinclair comparator nuts are a great idea if you have a way to accurately bore the holes in each flat. I've been meaning to build one, this might serve as inspiration.
I'm currently using pieces of 1" brass round stock faced square on each end and precision bored to the diameters I need for several calibres. They're a bit of a pia to make, frankly. And slightly overkill besides.
