Call CDI
If you have a question on something as important as how to cut luff, call CDI 978-922-2322. The problem in asking such a specific question on a forum is you may get an answer from someone who knows nothing about the subject, or even worst they know about a FF4 and you have a FF6. I am installing a FF6 and the instructions on "page 5, To assemble 1." says to cut the total luff 17" shorter than the pin to pin length of the forestay (they even have a drawing of the pin to pin dimension "L" so there would be no mistake." What I don't understand is what the "total" means, does it include the luff top fitting? Real test is you have to be able to raise the luff to get to the turnbuckle adjustment (hence the 17".)