I've always taken the formulas and tables as interesting little factoids. There are way too many variables in our little furnaces to make the formulas useful. Size of furnace. size of burner tube, exhaust, internal restrictions, back pressure...Etc.
I just try something, if it works, great. If not, I try something else. I once calculated CFM of different blowers by blowing up a 50 gallon trash bag and then converting it to cubic feet per minute. It was really useless, some would blow up the bag and stop, others would blow up the bag until it burst. Volume and pressure makes a big difference. Too much air and you blow all the heat out of the top of your furnace, not enough and there's no heat. It's all a balance that matches your particular furnace.
Really, it can take all the fun out of flames and melting stuff and things.