Trail and error may be the best. Use a tyre pressure gauge. You should be able to feel how compromised you handling is, especially cornering on the road, and get a good understand of the lowest pressure you can run on the road without compromising your cornering.
With those types of bike you may be able to work out a range of pressures for different conditions, eg sand, mud, rock.
I have a 2.35 gravity mud tyre on the rear and run it around 30 psi as a good road and off road compromise pressure.
Basically I pump it up to 30psi, and am in the habit of a quick check, squeezing each tyre before a ride. The back will slowly lose pressure over time, which makes it better off road and not so good on road, and when I can tell it has lost too much pressure, or feel the handling is too compromised for the road, pump it up again.
Keeps thing simple.