All of that makes perfect sense to me now. When the plane is originally designed, the FRL is somewhat arbitrarily chosen, but has to do with setting up some sort of measuring convention so that the numbers will be convenient to talk about (e.g., all positive). The FRL is indeed a useful and convenient line from which to make measurements. It can also be chosen based upon the attitude of the flooring during level flight. Right?

Thanks to all and a Merry Christmas!

Marl