measuring for wheel and tire

I would draw out a diagram, and as above. Don't forget that the wheel well for lack of better description, tapers towards the top. Don't forget that there IS some flex, hard cornering, etc

The reason there is not a "hard and fast" answer is that first, OEM axles are different over the years, and 8 1/4 are typically different width than 8 3/4

Many of us have swapped one for the other, newer for older, etc, so "what the car was" "is not what it is now."

In my case, I run a Lincoln Versailles 9" so nothing is "as it seems."