Rule of thumb for focallength in config.txt seems to be erroneous

[TCH68k]

If the original focallength was 128 for resolution 320x256, then it should not be renderwidth/2, but renderheight/2.

I've tested it via setting rendersize to 640 360 and focallength to 180. Works. (Also this setting makes pixel perfect upscaling to 1920x1080 if switched to fullscreen.)