My external monitor doesn't work at full resolution with my MacBook

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 1680x1050 @ 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 800x600 when my Mac says it's displaying 1680x1050. 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 1920x1080 so that would appear to not be the case here. The options for the resolution in Displays all work perfectly (various ones from 800x600 up to 1400x1050 when Alt-Scaled) but then it jumps up to 1920x1080. How can I add 1680x1050 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!