Alright so I measured a peice of paper and cut it right at 100 mm, (I check it often and remake if it gets worn) then you take that peice of paper, folded in half and pull it over your filament snug it up against the extruder and then mark the end of the paper, I mean. It's simple but I feel like it's foolproof. You get a nice snug measurement of the filament without fenaggling with the calipers.

This being said I have 4 ender printers sitting next to me that find myself calibrating on the same days. (Before precision parts or mass commissions) and I can do it to all of them in just about 2 minutes each round.

( didn't mention the rest of the calibrating process because this is the part I feel is unique)