I do the same because, as you said, it looks fine on my monitor. That said, for my site I don't figure it matters a whole heck of a lot because most of my visitors are on mobile devices anyways.
Mine is just default, too. Never thought to tinker with it because it works fine on most screens that I have tried (13" to 27" computer screens with various resolutions, several phones of various sizes, a couple tablets).
my default is also 1200px, but I have a secondary with 90% for the ones, who want to have it. Most styles works with this. I optimizes this a bit with also a wider messageinfo area, and some other tweaks to use the space.
I just added an option for shrinking forum for users. By default, forum expands to all screen width, but if user need, they can "shrink" to 1200px. View attachment dp1oQyqDSg.mp4
Option value automatically syncs with all opened tabs.