[Pete W]

Depends on two thing's, one how high do you want to lift the car and two how much of the ride do you want to compromise.
I get perfect lift distance on my car and never run above 80 psi. The ride is very nice.
Do not drive it at below 10 psi or you risk blowing the bag which like a sap I did at the start of this year because I didn't check them before I took my first ride after sitting all winter, that was an expensive lesson!

 

[Pete W]

I have mine a touch higher than the front but I agree I don't like the way high jacked up look.
Set it where you like it and see how it rides.
The actual psi setting will vary from car to car based on weight and suspension so there is no magic number as where to set it at.