I'd never heard of pixel clock before you mentioned it. Thank you.
I find two definitions, one in NVLinuxNotes4191.pdf which says
RefreshRate = PixelClock / ( VertFrameLength * HorizFrameLength)
[goes on to note "The VertFrameLength refers to actual frame length and not simply the visible resolution". (p 84).]
Assuming VertFrameLength equals visible verticle resolution, and PixelClock=215, I should be able to get a 2048x1536x24@68Hz. Not bad!
Only thing is, the other definition, courtesy of the XFree86 Modeline Generator at http://xtiming.sourceforge.net/cgi-bin/xtiming.pl
disagrees and demands 2048x1536x24@45Hz. I'm not sure why they're different, does anyone know why?
Anyhow, I went with the Xfree Modeline generator because it was more conservative, and because it generated cut'n'pastable configuration for XF86Config:
Modeline "1664x1248@64" 215.00 1664 1696 2512 2544 1248 1273 1286 1311
Modeline "1792x1344@56" 215.00 1792 1824 2640 2672 1344 1371 1384 1412
However, X doesn't deign to use these, putting this message in the verbose log:
(WW) NVIDIA(0): Not using mode "1792x1344@56":
(WW) NVIDIA(0): horizontal sync width (2640 - 1824 = 816) greater than 256
(WW) NVIDIA(0): Not using mode "1664x1248@64":
(WW) NVIDIA(0): horizontal sync width (2512 - 1696 = 816) greater than 256
Grr... in other words there are some constraints that this video mode calculator doesn't take into account. I have to invest more time to grok what they are.