OK, so I run BT090 and I don't know how much this does and doesn' translate to other tyres. Oh! And I'm talking track only.
In most (relatively hot) conditions here I run 1.8 (metric). When it gets really bastard smoking hot and/or I want to conserve tyres I'll got to 1.9-2.0, but only in practise sessions. Personally I ride pretty hard these days, but my bike is tuned for reliability (56.5 RWHP, but I'm aiming to move it to 60 not too peaky) so I get through two front tyres for every rear, and have little reason to go above 1.8 (maybe 1.9) in the rear.
Rain ... a different matter: in the rain you want more pressure (counter-intuitive) maybe 2.1max (keeps the tread open to shift water), unless there is light rain and the track doesn't have a layer of water ... then if you can ride hard and keep the tyre hot a lower (1.
pressure works.
The main point about tyre pressure is not contact area (increasing contact area gives you more roll on the tyre), but the key point is tyre temperature. Hysteresis is a property of your tyre. That means when you bend it through riding, you put enrgy into the tyre, then its elasticity springs back, but not as much as you bend it. The difference in energy between what you put in and get back goes into heat. So the lower the pressure, the more you deform the tyre, so the stronger the effect of hysteresis, so the more heat goes into the tyre.
You want the tyre to be the right temperature, and different compounds are chosen at different hysteresis rates. For us (mostly) we have only one compound to choose from (at the race tyre level, e.g. BT090) since we don't have the power that the bigger bikes do.
Another important factor is how long tyres last. Often what determines a tyre's life is not so much how much it gets worn, but how many heat cycles it goes through. The rubber has oils in it which are exuded and sticky at temperature. If you get it up to temprerature too many times all of these sticky oils come out of the tyre and you are left with a hard dry tyre. Again we have to worry far less about this than the bigger bikes since we rarely will be really gettin gthe tyres hot. A 600-1000 bike ridden hard enough can cycle a tyre only 3 times before rendering it useless. I usually find that the BT090 on the NSR wears down before it looses stickiness.
So if in doubt -for the track- choose 1.8.
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