How do you figure out what sort of boost you'll run?

I know that my jrsc pulley, combined with a 5" crank pulley will warrant 9psi on my setup.

With that info, can I figure out what running a 4.5" crank pulley will do for boost?

Is there any sort of equation to figure out what kind of boost numbers I'll see?

Also, what are the oem crank pulley sizes that work on the GSR motor?

Thanks!