That'* a very good question.
Use this chart to see where you'll end up. Your project car is likely to be a higher compression ratio than an OEM supercharged car.
Simply find your SC (intended) boost level on the vertical, then draw a line over to where the compression ratio of your engine (diagonal lines) meet. Then draw a line straight down to see your effective ratio.
In my case, I'm at 11psi of boost at WOT, and a compression ratio of 8.5:1, so I end up at an effective compression of about 11.2:1.