whats the correct back spacing?

I have a 68 dart that is mini tubbed, and leafs relocated. I currently have Crager SS 15x8 & 15x7... I bought them off craigslist NIB which they are but I dont like the way they fit on my car... a lil to close to the fender lip for me. I still have SBP but I really like these centerlines but sont know the correct offset.

http://www.centerlinewheels.com/wheels_detail.php?mw_id=34&sw_id=660

http://www.centerlinewheels.com/wheels_detail.php?mw_id=34&sw_id=365

http://www.centerlinewheels.com/wheels_detail.php?mw_id=34&sw_id=371

http://www.centerlinewheels.com/wheels_detail.php?mw_id=34&sw_id=364