Lots of people will disagree, but if you add more -ve then +ve pressure, all it does is drag the fork down a bit into it's travel and equalize the pressures anyway, so I reckon it's a waste of time.
Why do you want to run a maximum of 30mm sag? I usually reckon on running 25% sag, which is 35mm on your forks, but it's very hard to set be that accurate anyway.
What I would do, is set 30-35mm sag for a start, keep +ve and -ve pressures the same, then see how it goes. If it feels too stiff, drop 5psi out of both chambers, too soft, add 5psi, and so on. I've found different forks work better with different amounts of sag
FWIW I think people get too hung up on the 'numbers game' with suspension. TBH I couldn't tell you waht settings I'm running, but I undersand how the settings work and how I arrived at the set-up I use and it works just fine. 🙂
One tip – When messing with pressures, when you adjust the +ve pressure, let the -ve chamber down first as you run the risk of a big imbalance of pressures which will drag the fork down if you don't
So, to make a change
Let out -ve
Adjust +ve
Reset -ve