Drive a solid peg into your lawn and hook the tack over the peg. Pull on the head (or clew) when measuring... that ought to be close enough. If you are so close to the max luff length that your furler will take that measuring stretch is a factor, the sail is too big.
__________________
It was a dark and stormy night and the captain of the ship said.. "Hey Jim, spin us a yarn." and the yarn began like this.. "It was a dark and stormy night.."