There are 4 more PSI pushing on the outside of the tire at sea level, I think it's the other way around.

 

The recommended pressure listed in the owners manual is a good place to start.

If you've changed tires to something else, check what their recommendation is for pressure. It may well be different than the oem pressure for the oem tire.

If you're into tuning for specific reasons, use a note book and a set circuit to test on. A pretty big circuit, so you can feel high speeds, rough surfaces, various curves, etc. Make 2-3 psi changes, note them, ride it, and note the results. Go up and down, until you find yourself going "yech" and stop there. Fine tune in between those upper and lower pressure ranges.

Very generically, a higher pressure will give you a harsher ride and sharper handling. A lower pressure will give you a softer ride, but mushier handling.

Since you haven't come back to this question you raised, this is all probably moot. But, start with the owners manual.

 

I explained it in my first entry, if you choose to disregard that, fine by me. Just remember that when the outside and inside pressures are the same, the tire is effectively flat. A flat tire at sea level has 14.7 psi inside and outside. To support any weight, the tire must have more pressure inside than it has outside, the bigger the differential, the more weight the tire can support (up to the manufacturer's design limit). That differential can be increased by either pumping more air into the tire OR moving the tire to a location with lower OUTSIDE pressure. (Higher altitude) The differential again decreases as the tire is moved to a lower altitude (higher pressure environment)