That might be true, but he said he had a 5.5" wheel with 3.5" backspacing, and wanted to know what the offset was. If that's what he has, that wheel has 0.5" offset.
How was the half inch figured out, I was always told 5.5 inch rim means 6.5 inch total width of the wheel. Cut that in half and get 3.25 inches, that would be a zero offset. Because the wheel has a backspace of 3.5 inches, a quarter inch more than the 3.25 inches, it has a .25 inch positive offset. This is just how I was taught to figure it out, It makes sense to me but maybe I have been wrong all these years.