But how do you actually make a new custom screenmode?
When I open up my screen preferences, I am given a bunch of screenmodes depending on the monitor drivers in my DEVS folder. I can tweak these, but if I choose SAVE it writes my new tweaks over the original settings for that screenmode.
There is no "SAVE AS" to create a new screenmode from within Screenmode prefs.
I was trying to say that the Amiga's 640x400 native mode is pretty much 16:10 aspect which is what most PC Widescreen LCD monitors are ie 1920x1200 not 1080p. You don't really need to create a 640x400 screen mode it exists already surely.
So the issue isn't so much the resolution, but the workbench icons being designed for tall pixel modes like 640x256. If you look at GEM on the ST the icons are designed for 640x400 so in 640x200 they look tall, just like Workbench icons on classic WB looks fat in hires interlace.
So my advice is new icon set and setting workbench to 640x400 NTSC mode and a SCART connector.