Backspace is measured from the outside lip of the rim to the wheel mounting surface, so it includes the 1/2" of extra width outside of the tire mounting surface. An 8" rim is actually 9" wide outside lip to outside lip, it's 8" from tire mounting surface to tire mounting surface. So an 8" wide rim with a 0 or 1mm offset has a 4.5" backspace, an 8" wide rim with a +13mm offset would have a 5" backspace.
both of the above is true, but you order 8" rims from jegs or summit or anyone else, and they measure from the mounting surface to the inside of the wheel where the bead seats. at least that's the way I`ve bought them for 40 some thing yrs.