Need to know what chassis you're running there.
Most have a width adjustment coil and also a centering adjustment via a jumper on the board.
^^ This. You can pretty much just use the default settings. then twiddle some pots until it looks good. It is probably all you really need for an arcade monitor.
There are ways to tweak CRTEMU settings to get the mode geometry looking right, from adjusting monitor presets prior to generating/installing modes, to fiddling modes directly using arcade_OSD, even slider controls available in-game. but these are more useful for people using CRT TVs. Unlike arcade monitors, TVs usually have very limited, or no, provision for changing the screen geometry.
If you really want to down the rabbit hole, read the pinned thread on monitor presets. Once you get these right everything else falls into place.