Recently I replaced two 19" outboard monitors with two 22" widescreens.
macOS provides a utility to set the arrangment, however there is no way to know how well you actually line them up (I wanted the bottoms of the two outboards to be at the same coordinate)
Norman provided an idea which worked out quite well, and I have expanded on it.
Here is Xojo source code for a VIEWER (you need to use the system utilty to change them), but it reports all the relavant information
This reminds me, I had/have a problem with macOS not remembering the resolutions and placements of my external monitors across restarts. I have a three-monitor setup similar to DaveS, running off a MacBook in clamshell mode, but when I restart it forgets the way I set it up, e.g., the left monitor will flip to the right and the right to the left, the side monitors will switch from 2560x1440 to their native 4K, etc.
It was extremely irritating until I found this little utility that solves it. Now I just execute this command on restart, using a Keyboard Maestro macro, and all is solved.
