I echo all these considerations. Getting an optimal heat treat is not to me necessarily important from an hardness aspect but primarily metallurgical. If you get a matrix of soft structures at the edge(retained austenite or pearlite etc) and or large grains, different wear rates can result in a toothy edge quicker. Great for knives not so much for a razor, but even this is no big deal as long as you don't mind touching up your edges more frequently. You can get a perfectly serviceable edge in the 50's rc though I will concede there is a sweet spot which is a matter of opinion to it's rc value. Some of the customs I have made I can hone as good as my best fili. The difference is my filli is so hard I can go weeks sometimes longer before I need to touch up. In fact I've toyed with the idea of cold forging a blade from a lower carbon alloy to see what kind of edge can be achieved through work hardening alone, this will be a future experiment. My forge kit is dead simple though I do know the blade temperature at any point in time (not the same as control). I still like to squeeze as much as I can out of the process for the reasons I've outlined here.