Wheels & Tires - how to measure wheel spacers

Bought c6 wheels and during install the tire is hitting the bump stop. How do I know how much I need come out? Its a 19x10 I put 305s on the rear.

Whats the offset?

68mm...diameter of tire is same as stock. Think I need an inch spacer. But not 100% sure.

On a 10" wide wheel, a high 30mm/low 40mm would look perfect. In your case a 1" spacer (68-25=43) would have the wheel/tire damn-near flush with the quarters without hitting them on huge bumps. Get your fenders rolled to be safe