Until everything gets transferred to the new tread - the MSNZ Historic regulations for roll bar height is 50mm above the helmet - not 100mm - Appendix Six, Schedule AA 4.4 (3) (c) "Open Vehicles - the height of the main rollbar should be at least 50mm above the top of the helmet of the normally seated vehicle occupant".
The question is what is a "normally seated vehicle occupant"? A driver who is 5' 2" tall or one who is 6' 2" tall? Do you need to rebuilt the height of the rollbar every time a different sized driver gets in? Or do you build one for an average sized person (say 5' 9") and leave it at that?