Maybe you need to stop typing and read the material. It's really very basic and well explained. Yes, the measurement is between the film and lens planes, but as you say being really precise or accurate isn't really necessary. Jim's "cut-out thing" is in more than one of those links. They may be round instead of square, but same thing.
I was assuming it was some kind of PDF you could print and cut out that's what I was looking for so I saw the pages with math on it I just didn't bother :-0
~Stone | "...of course, that's just my opinion. I could be wrong."~Dennis Miller