You're confusing me. The backspace you state is 3-5/16 inch with a -30mm offset. Then you say the offset is just over 4". Which is it? A negative offset will have a backspace of less than 1/2 the wheel width.
Keep in mind that free advice can frequently be worth less than you pay for it.
Its because of how the measurements are taken. An 8" wide wheel is 8" from bead to bead, not from outer edge to outer edge. Most 8" wheels are right at 9" overall width. The backspacing is measured from the outer edge on the back of the wheel. So when you calculate backspacing from offset you have to 1/2 of the overall with, which is going to be about 4 1/2. Also, you have to add the offset to 1/2 the wheel width, not subtract.
True but as general formula its easier and more consistent to add the negative offset. Then the formula stays doesn't change based on whether the offset is positive or negative. But no, you weren't wrong in how you did it