I think a note should be added somewhere (in the TODO list or in the release note) to explain that videl borders are working well for falcon resolutions, but not perfectly for St mode video. (the screen is not well centered in st medium or st low resolutions). (it should be at least a "to fix").
But whithout more infos, I can't guess the correct algo, so, for now, I leave it as is.
Can't you just use code for ST emulation? I guess there isn't a single demo using ST LOW/MED so it's more like an issue for GEM apps, i.e. it doesn't matter what timings are, just how does it look in the end.