Hoist the tape measure up using the genoa halyard attached to the head swivel and the tape attached to the sails normal attachment point (make sure to add a length of light line so you can get it back down again without relying on the tape alone!). Hoist all the way to the top – effectively the absolute maximum hoist distance – and measure to the tack attachment shackle, and make note of this dimension.
For the sail luff length, make a 2% deduction – so if the max hoist is 10000mm 2% = 200mm so sail length will be 9800mm. This deduction allows for the head and tack webbings and potential stretch so it puts the head swivel in the optimum spot for furling. On furling sails (mains and genoas) we fit soft webbing loops at the head and tack, these are around 50-60mm long each.