Anyway after reading a few articles on rebuilding the forks I did the mod to the top nut to allow me to do oil changes without stripping them down, new progressive springs (replacing the same that were fitted but I didn't know how old they were), new guide bushes, seals etc, the full monty
Using a 100ml syringe, washer pipe, alloy tube and a drill bit depth lock I was able to full the forks and siphon off the excess leaving the factory recommended air gap when compressed and no spring. However once rebuilt and spring installed etc I removed the new bolt that will allow me to change the oil in place and dropped a threaded rod inside with a nut to stop it falling right in, this allowed me to then measure the distance/air gap for the oil when the forks have the correct amount of oil in but doing so with springs installed and fully extended.
I got a measurement of 385mm from the top of the fork nut inside the 17mm hex. Now knowing this if I want to change the oil for whatever reason I can now add oil and get the right air gap as if I was doing it with no spring and compressed
Hopefully this maybe of help to someone with a pre 1995 bike