How do I find out the the right offset for custom centerline wheels 18x9.5 (rear) for 1986 Fiero GT?

Im trying to buy 18x8,18x9.5 Centerline Sundnce Galaxy forged wheels for a 1986 Fiero GT. Centerline says I need to find out the right space needed between the callipers and wheels befoer I can place the order. I thought they should know that stuff. Are they talking about the offset? Can anyone help out? Thanks