For on-road use I have been working to a formula I heard yonks ago, which has served me well. Can't remember where I heard it - no surprise there. I measure tyre pressure when cold, then re-measure after a run long enough to bring the tyres up to 'operating' temp. I'm looking for a 4 psi increase. Less = over pressurised tyre, more = under pressurised tyre. Intuitively, this makes sense to me. Apparantly, the 4 psi increase is largely independent of load, so for any given load the tyre pressure that results in a 4 psi increase is correct. I have never run at max pressures - 28psi front and 32 rear, cold, on 235/85's generally works for me with an empty wagon.
1982 LHD RRC - gone. 1986 LHD 110 (300tdi auto) - gone. 1995 Disco 1 - still going. 1996 Disco 1 - scrapped. 2002 90SV build up (300tdi auto) - in progress...