Just a wild thought, have you used the rigging calculator in the SBO store? It's a start but a boat that is 30 years old with how many mods by how many owners will
Most likely fit any calculator as a best guesstimate only.
Another way is to figure it out. What is the purchase on the mainsheet? Mid boom or end boom? Distance mainsheet travels to get to the helm. Add it all up and add 20%
For example, my islander Freeport has end boom sheeting, 3 to 1 purchase and travels to the mast down to the cabin top and back to the helm, full extension when running on a 14' boom is about 20 ft X 3 is 60 ft + 16 ft boom + 3' cabin height + 16 ft back to cockpit = 95' X1.20 = 114'. If it is too long I can always shorten it.
It's not hard but requires some thought
God luck,