Actual wheel width

Backspace is backspace. It's measured from the outside lip of the rim to the mounting surface, so, there shouldn't be any reason to add anything to it.

Now, if you're planning on installing a 15x9 you do need to remember that rim is 10" wide outside lip to outside lip.

If you're trying to figure out the clearance to the spring, for example, I use the tire section width, backspace, and rim width. So, if you have an 11" tire section width, subtract the overall rim width and divide by two. So, that's a 1/2" in this case. Add that to the backspace, plus whatever clearance you want to have. Then compare that to the distance from the spring to the wheel mounting surface.