Mine are 275 wide. I run 28psi, as its whats specified in the door
I know you have changed your tyres, I dont know if there is a formula, but i guess weight will come into it too. Do your tyres wear evenly?
I think around 28psi is good for normal driving.
When I do a g-tech run trying out my 0-100mph, I put my rear tyres down to20psi and soften the rear suspention and it really it increases grip dramatically
As your tyres are wider than mine it will work out a simular weight per inch on the tyre.