Rectangularity of Parallelograms

A rhombus is a parallelogram with perpendicular diagonals that in certain circumstances is a square (and thus a rectangle). Can you devise a measure of rectangularity of parallelograms that allows you to tell how much more rectangular one parallelogram is than another? Can you devise a second and different measure of rectangularity of parallelograms? Under what circumstances might one such measure be preferable to the other? [See the "Polygon Explorer" and "Polygon Synthesizer" applets on the mathMINDhabits website]