is there a basic mm of the fork leght you should use under braking? ive found the sag should be about 18/20mm so will see how far that is off
Depends how hard you're braking and whether it's on the flat or going downhill so it's not really possible to say.
Static sag (you on the bike in a normal riding position and at riding weight) you could try setting sag at 20-25% of the fork travel.
If they're 120mm forks then I'd be starting at around 25mm static sag.
That's about all you can do externally on those forks as you have no separate high and low speed compression damping and if you increase the overall compression damping in an attempt to prevent brake dive then they'll lose compliance over chattery type stuff
Anyway, if they still seem too linear, and blow through their travel too easily under braking, then you could try fitting a heavier spring and/or reducing the volume of the air spring chamber (by adding oil) in order to increase the air spring rate - make it more progressive, in other words.
Think of something like only 5-10ml initially, as you want to reduce the air spring chamber volume but still leave room for air at full compression, obviously.