I had a new main made last year and it was really just a case of unscrewing the track from the clew of the sail then re-attaching it to the new one. I had the same question that you have now but in the end it didn't matter. The sailmaker should know how long to make the fiberglass "bolt rope" that the track gets screwed into BUT... my track is 28 inches if it helps.. the glass rope is longer by 3 inches or so.