Thanks gentlemen.
Here's the deal, I'm want to stop 175kg of bike and 100kg of rider from reasonably high speeds.
I had fitted a 50mm Marzocchi front end, but being a pedantic old bastard, decided that it wasn't even close to being period correct and I'm going back to the XR front end for now.
Sounds like another master cylinder might be the best option to keep it all looking 'right' until I can get the front end I really want.