What should the psi be set to on 18x8, 225/40r18 ?

I use 32psi all round on my 18s, which is about what the standard is, I don't see any reason why you should change the pressure greatly given a different rim size. I guess though you should be more aware of upping the pressures if you load the car up, to lessen sidewall deflection, especially so if you take it near a track, e.g. 18th June, Bruntingthorpe

I find the required pressure varys more with the tyre than the size... for example...

225/40ZR18

Pirelli P-Zero Rosso - favorable pressure 36 - 38psi all around

Vredestein Ultrac - front 31psi, rear 28 - 29psi

Goodyear Eagle GSD3 - 31 - 33psi all around

So basically, stick them at the recommended pressure in your handbook, and tweak them from there!