I measure it from where my fat head is parked. Trying to do all that math with the variety of barrel lengths I own is just too much math.

Lets see, my 686 has 30 inch barrels on it, the length of pull is 15 inches and the trigger sits about 2 inches behind the breach face, so... Wait, need to get that 2 inch estimate just right, so have to get out my calipers and measure that distance. Do you see my point. If not how long is your arm to the end of the muzzle on that handgun your shooting.

Generally from where I parked the car , to the target. but ! I always add , "give or take"