Offset and backspace are related, but not the same. Backspace is the distance from the rear flange to the mounting surface. Offset is the distance from the centerline to the mounting surface. The offset of a B-body wheel is 6mm.
To find the offset, measure the flange to the mounting surface from both the inside and outside flange to the mounting surface. If the rear is more than the front, you have a positive offset of the difference. 6mm is about 1/4 inch.