Offset and backspacing are two views of the same geometry
Offset is measured from the wheel centerline in millimeters. Backspacing is measured from the hub mounting face to the inner wheel edge in inches. The nominal conversion uses wheel width divided by two, plus offset converted to inches.
The second result adds one inch to nominal wheel width before dividing by two. That approximates the extra outer-lip width commonly present beyond the bead seats, but real wheel construction varies.
When brake or suspension clearance is tight, measure the actual wheel. Published backspacing from the wheel manufacturer is better than a generic lip estimate.