Every speaker and every room is different. In my 50+ years of audiophilia, I've never had a room that didn't have irregular measurements or opennings that would defeat this extremely naive measurement concept.
I appreciate the attempt to help with the second statement, Dave, but would you expand a bit on why you've included the first (quoted above)? I read back over the responses here, and see that most of them are basically saying what I suggested; that there are only very general guidelines to assist you in getting started and nothing so specific as you seem to indicate as being a sign of being "naive". Sorry if I'm missing something here or have misinterpreted you. In general, I agree that you should let the room and your ears determine where the speakers go, but it is a whole lot easier with a place to begin, and if you are taking notes and changing things a measurement or two could help keep track of things. That or simply putting some masking tape on the floor with notations. The OP's question is pretty simple though, and was answered straight away by Newbee.