Those round bottom scrapers are fantastic to work with - restocking an 1850's period double 10-bore, and they are doing a great job on a piece of rock-hard English Walnut.
I also have a couple octagonal scrapers from the same people, and they also work very well.
Yeah, measurements are obsolete, just use the old Mark 1 Mod 1 eyeball. And maybe a straightedge from time to time.