Setback measurement is somewhat arbitrary and is determined by the mid, motor type (mass), and boat with load and distribution. Although the the mid may or may not have an offset, as 90 5.0 said, a longer mid travels futher during positive or negative trim. Under neutral trim, all things should be equal. The fun thing is that the weight of the engine overhead is the same, more or less, on any setup. Likely though a 15" mid will have the motor lower than a 20" mid by 5" - but the mass doesn't change concerning setback. What does change is the point at which thrust is applied.
I probably could draw this better than describe it, but we'll see... If you trim 5 degrees as an example, the radius of the arc (LU to pivot) determines the thrust point (use 1 radian as an example - google it). All that being said, you need to figure out what works best for your boat since they all are a little different and probably no 2 boats weigh the same.