Most of the time if I can figure out how things were done 100 years or more ago that is the best way to do it now. There is always a but....Flattening the back of irons may be one of the buts. One of the reasons I will replace old irons with new ones from Hock or LV, if they are available, is the vast majority of old irons will have evidence of a previous sharpening where that day's "ruler trick" was used, whatever they called it then.
What brings this to mind is for the last couple of days I've worked, as long at a time as I could stand, to sharpen the irons of a 1/2 set of hollows and rounds. All of the backs are bellied back from the edge as if the ruler trick was used and of course there is no way to buy new replacement irons. I have a choice, use the ruler trick myself which I have an aversion to and would condemn to hell (if there were one) anyone that does that to an iron or beaver away at getting the back flat. So far beavering is winning but as the FSM knows I've looked with lust at my small metal ruler. May she strike me dead.