Thanks for the compliments. I was targeting "optical centering" which raises the image a little bit above the centerline, purportedly adding weight to the bottom (so it doesn't get hung upside down!) It can be done graphically, or I have used a handy html program from this guy (he shows the graphical method too). In all honesty, I am beginning to think I might lean toward positioning just a tad lower, some of that may come from whether or not one adjusts the calculated value for the overlap of the frame.
I should add, I have seen images that were almost panoramic horizontals mounted in a rectangular frame held vertical and with the bottom of the image almost on the centerline. There seems to be some occasional benefit in breaking conventional rules to achieve an effect. Somewhere years ago I heard of a competition that specified all work was to be in 16x20 frames too!
I occasionally frame square, but since I don't sell much work, I've lately made 16x20 a sort of standard that works for most of my photos and on the rare occasion, a quarter-sheet watercolor so I can recycle frames.
Edit: Oops! Looks like I got antsy and jumped in without reading to the end, sorry for the redundant info.