Why isn't tyre pressure an exact science?

Since reading Norm C's post (this is all your fault ;) ) about skinny tyres, my faith in my tyre pressures went out the window. After numerous searches here and elsewhere I just don't know where to turn (sob!). I've always used Pirelli's Rule; check when cold and if they are 4 psi higher when warm then this is the correct pressure.This is why I have always run 32psi on an empty vehicle. If Pirelli's rule is correct, what is it correct for? Comfort, even wear or tyre load?
Then there's the manufacturer's reccommended pressures which in other posts has been explained as comfort (which is 32 for 275/70/16).
In Norm's post plenty of people run 235/85's at 45psi or more but I presume this is loaded up.
Mine are BFG A/T's which seems to mean that the brand makes a difference as well (references to stiffer sidewalls etc)
I'm thinking I would like my tyres to wear evenly so what pressure = even wear?
The BFG website has loads of info but nothing about pressures.
The Beaurepairs website in its 'tyre tips' section states "Always keep tyre pressures at the levels recommended by car manufacturers". Then in the same paragraph states "Using the recommended high speed/full load pressures will provide improved safety and wear for day to day driving". ??? That's 80psi for my tyres!
So what's left? Experimentation. Run your $1000 tyres at whatever pressures you think are right then when they start to wear unevenly you adjust but will still have more wear on the already worn part.
So I'm running 40psi. Why? I think they need more than 32 but less than 50. How do I know that? I don't; it's guessametrics. It seems that for all the research into tyre construction/handling etc when it gets into the consumer's hands there is no difinitive information on what to do with them except put them on your wheels.