I took a look at your gallery to understand what optically centered meant, as you used the term. Looks very nice! Can you explain what the formula is for such centering? I note that several of your rectangular images (Soaring Arch, Trail Junction) also appear to have been optically centered -- same formula?
Aha! Now I'm in trouble!

Thanks, yes I've been using a Javascript some guy had on the web. And after the initial efforts about 10 years ago
Ye Olde Photographer has just been cruising on inertia! Actually, of late I have been using polypropylene photo corners to anchor the print so I can not only recycle the frame, but also recycle the mat, for exhibition purposes. (At latest purchase, museum mounting board to make a double mat and a backing piece represents about fifteen $US just for matting. I have a mat cutter and cut my own.)
Ah -- here we go --
Russell Cottrell (Glad he still has that out there.) I've played with a copy of the script trying to set it up to handle my double window mats (and learned I know just enough about such scripts to be dangerous!)
Since about 97% of my framed prints are the same size print and frame, I just keep using the same numbers. I make the reveal on the bottom wider than the sides and top so I can put my signature on the inner mat instead of the print. The script had some checks for conditions where that centering produces weird results and it pops up a recommendation to use equal margins or whatever.
That linked page also shows a graphical method. Note the page also has some weasel words about that's a starting point and one may want to adjust slightly under some conditions. The main theory appears to be having a wider bottom section adds "weight" to the presentation; the print center is slightly above the midpoint of the frame. That is, you probably wouldn't want the top margin narrower than the side margins, but you could just slide a print up and down and see what you like.
I usually print about 10 5/8 inches square on a cut piece from 11x14 paper, trimming about 2.5 inches off the 14 and using it for test strips. The print gets mounted behind a double mat with the inner window about 10 1/4 inches square.
Edit (now that I'm awake

):
I only now realized that you were apparently looking at my PBase galleries, vs APUG. Soaring Arch and Trail Junction are 8x10 prints (contact prints via my 8x10 pinhole camera!). They are in a 14x15 frame and hit one of those conditions where using top and side dimensions equal came into play; e.g., maybe not quite fully optically centered per that method. But as I recall, that script flagged that condition.
Hope that helps.