...backspacing is basically how far back into the wheel the center point of the wheel diameter sits and offset would be essentially where the barrel of the wheel sits in relation to the centerline of the wheel? If that makes sense.
It is all related to the mounting surface of the wheel.
Correction - see Blu’s post below...
Back spacing is a little difficult to calculate because it is between mounting surface of the wheel and the outside of the lip of the wheel, while the wheel width is actually measure from inside to inside of the lip. And because nobody lists the thickness of the lip, I just use 1/2" as an assumption.
The other measurement is offset which relates to how far the mounting surface is from the centerline of the wheel barrel. Positive being away from the centerline of the car and negative towards the centerline of the car.
So, your 7" wide rim with 38mm of positive offset should calculate out to 5.5" backspacing assuming 1/2" for the wheel lip:
3.5" (half the rim width) + 1.50" (offset in inches) + 1/2" (lip thickness) = 5.5"
In the case of a negative offset, subtract the offset rather than add it.