Apologies for the wording, but this kinda sucks
. The old FlatPanelProperties option was waaaaaaay simpler, it didn't require any calculation. It was just one setting and you were done, regardless of in/out resolutions.
Open source xrandr drivers can do this, an example with Intel graphics:
xrandr --output LVDS1 --set "scaling mode" "Full aspect"
the other options being "Full" and "Center".