Backspacing confusion

Backspace is the distance from the mating surface ( part that touches the rotor stud face ) to the furthest inner part of the wheel lip ( where the wheel weight mounts on older wheels ) .
Backspace is a static measurement in relation to rim width.
For example:
A 17x9 inch wheel with 4.5 inch backspace has the same backspace as a 17x8 inch wheel with 4.5 inch backspace BUT the 17x9 has a 1 inch wider outer lip.

Offset is the distance of the mating surface in relation to the center line of the wheel. Negative offset is toward the inside of the wheel and positive offset is toward theoutside of the wheel.
Offset in how it relates to backspace will depend on wheel width* ( unless offset is 0 )
For example: fyi offset is usually expressed in mm not inches
17x9 inch wheel with -25 offset will have a backspace of 3.5 inches.
17x8 inch wheel with -25 offset will have a backspace of 3 inches

Google offset vs backspace to see a bazillion pics of the description above.