My understanding is that camheads have linked brakes so that when you apply front brake, rear brake is applied at 30% of front brake pressure.
What I'm wondering is that if I apply rear brake as well as front, is the rear braking I'm applying add to the system's 30%, or does the system back off its application of rear pressure as I apply pressure to maintain the 30% ratio?
It is a somewhat complicated brake system.
First I don't know where you got that 30% from but there is no fixed front/rear apply percentage. The ABS system is an adaptive system so it looks at front & rear apply pressures vs tire slippage then adapts the braking bias to keep the braking even, even if there is a rear seat passenger or a load change on the motorcycle.
(This is IF the braking is basically done using ONLY the front hand lever).
When the front brake lever is pulled that runs the rear servo pump to apply the rear brake (there is no direct hydraulic link between the front hand lever & rear brake).
Now the rear brake is a strange affair (or at least the hydraulic plumbing is)-- The rear brake is basically an either/or brake apply system.
If you ONLY use the rear brake pedal (alone) then you get a direct hydraulic link to the rear brake (no front brake & rear servo pump)
There is a pressure bias valve between the front & rear systems to keep the rear pedal isolated from the front calipers & to keep the front brake from blowing brake fluid out of the rear system with front brake apply.
In a simple explanation on how it works-- If you use the front lever alone then you get front braking in proportion the what lever pressure that you apply, the ABS computer then determines how much rear brake to add based on wheel slippage, pressure applied, spin down rate, etc.
As mentioned above when using rear brake pedal you only get rear brake but no rear servo assist.
The part that a lot of riders don't understand is the either/or for rear brake apply.
If you stop the bike from speed using the front lever then the servo applies (lets say 30 psi to the rear caliper), now you step on the rear brake pedal lightly (lets say 20 psi) then NOTHING is added to the rear brake apply as the either/or rear brake valve remains biased to the higher 30 psi front pressure.
Now without changing the front apply pressure, & assuming no tire slip, you step on the rear brake pedal harder (lets say to 40 psi) that rear pedal 40 psi is higher than the front's 30 psi to the rear so the either/or valve closes off the front servo input to the rear & opens the rear input (so now the front braking remains the same but you have increased rear braking pressure).
So now you find that you need to stop quicker so you squeeze the front lever harder & increase rear servo pressure to 50 psi. The either/or valve now closes off the rear pedal input as the front pressure to the rear brake is now higher than the rear pedal pressure so now it is back to allowing the front to control the rear brake. (note: that either/or valve is my name for it, that is not an official valve name)
If you want to do a little test just put the bike on the center stand, now have someone spin the rear wheel (or do it yourself using your foot)-- (you will find NO rear braking from front brake lever)
Now do the very same test with the ign key turned on, the rear wheel should lock up just as you start to spin it (that is the rear brake servo kicking in & sending pressure to the rear brake)
When in ABS lock-up mode things change as the front is accumulated & the rear make-up fluid is servo only.
(the above a is a pretty simple explanation of complicated braking system)