I've got a MacBook Pro (Early 2015, OS 10.11.2) and I've also got a Dell 2209WAf monitor, which I'm using with a VGA cable and this adaptor.
The monitor's optimum resolution is 1680×1050 @ 60Hz and RDM shows this as an option. However, when I try to use that resolution, my screen looks like this 
which is clearly not right (it looks impossibly pixellated and really hurts my eyes, you can barely see anything on the screen).
On the monitor, it says it's displaying 800×600 when my Mac says it's displaying 1680×1050.
I've seen questions about VGA cables/adaptors etc. asked before, but they all said it was to do with the resolution being too high. My monitor (and adaptor) can display 1920×1080 so that would appear to not be the case here.
The options for the resolution in Displays all work perfectly (various ones from 800×600 up to 1400×1050 when Alt-Scaled) but then it jumps up to 1920×1080. How can I add 1680×1050 to this list, as presumably that would display clearly? SwitchResX doesn't fix this and neither does RDM, I've seen stuff about editing .plist files but that was all for Yosemite, apparently it's different for El Capitan.
Any help is massively appreciated!
Best Answer
You can go to System Preferences > Displays, then option-click (press option key while left-clicking) on Scaled to expose additional resolutions that aren't exposed with a normal left-click.
Otherwise, you have a great choice of software for that, like switchResX: