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.
__________________
You can't oppress a people for so many decades and have them say.. "I Love You.. ".
"It is better to die standing proud, than to live a lifetime on ones knees.."
Self Defence is no excuse for Genocide...