After Market Rims: Backspace?

I want to put new wheels on my 95 Riviera. I am thinking about 17x8 rims and have figured out that I need a 5x114.2 Bolt pattern.

What I don´t know is the backspace needed and the diameter of the center hub.

Without me measuring and calculating wrong values - who can tell me the right numbers from his experience?

All help is appreciated - thanks in advance!