-12 offset with 4.56 backspacing What would be my total backspacing with that offset?

 

They're two different ways of measuring the same thing. Backspacing is the distance from the rear lip to the mounting surface where the offset is the mounting surfaces relation to the center of the wheel width. So to answer your question, the backspacing is just the backspacing that you listed, 4.56. Using the chart below and the numbers you gave though, we can determine that you're also talking about a 9" wide wheel.