I thought about the files too. They are in 5-C scale increments. I don't think that tells you much. In the knife blade there's a huge difference between 55 and 60! I doubt you can get more accurate than 4 or 5 points with those things.
Repeatedly using the same file is probably just as good. Files are just above where where want things anyway. If you use the same triangular file over and over, you'll get a feel for how that file acts on annealed steel, hardened untempered steel, and tempered steel. (I know I did.) Probably just as accurate at the file set. In otherwords if you use your 'testing' file just after quenching and it skips good news. Then after tempering it should bite, but only a little - good news.
However, if you want to know if you got 56 or 59 HRC you're gonna have to have a tester.
Just my thoughts,
Steve