Hi Peter

A bit more context would be helpful.

What type and size of mast?
What type and size of sail?
What type of yacht?
What ground are you sailing on?

Ben,
Its a windsurfer sail that has been recut once already, I think the foot has been attacked, but is an awful fit on the mast. 30 Kg is a fair bit of weight, I will try.
And you dont add any extra luff curve. Its on my new to me LLM.
Peter

Set up your yacht with mast boom and mainsheet, fix a rope from the top of your mast to the end of your boom. Tighten down your mainsheet and tie off. Set up a line from the top of the mast to the base on the inside curve, measure the distance from the straight edge (the line) to the mast every 100mm. Get some long strips of card and transfer the coordinates finally cut the luff curve and use it on your sail. PS mark the boom height.

Thank you for sharing the information with all of us.