@nubechecorre
I'm sure someone will provide the mathmatics but I remember when I wanted to create a weird mode (I think I wanted 640x512) I simply went into the Picasso96Mode prefs... selected a mode very similair to it and started experimenting with the editing of it... eg you get a test screen and keyboard controls to move alter the viewing size and location...
good luck
of course, make sure you have your card attached otherwise there'll be no modes for you to edit in this interface.
there's other buttons like interlace and doublescan, not sure if I toggles any of those however, probably unlikely though...
no sure what my 0S4 classic is operating under now... I think ultimately I gave up the pursuit to use as much screen as I could without stretching it... PAL is of course 512 where NTSC is 480... well, roughly in those resolutions I think...
Got in to those sorts of things when I used DCTV... any TV work I done I was using 736x566 overscan or something like that... and NTSC was smaller...
ANYWAY!