display adapter not showing up in resolution

Where Apple don't develop plug-and-play drivers for monitors (I assume) and hadn't added a profile for mine, it couldn't recognise it. Windows evidently could read the data just fine, which is why it worked perfectly every time. Okay, so I managed to work out what the issue was in the end - it was OS X's handling of the EDID data from the monitor. plist files but that was all for Yosemite, apparently it's different for El Capitan. 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. The options for the resolution in Displays all work perfectly (various ones from 800圆00 up to 1400x1050 when Alt-Scaled) but then it jumps up to 1920x1080. My monitor (and adaptor) can display 1920x1080 so that would appear to not be the case here. asked before, but they all said it was to do with the resolution being too high. I've seen questions about VGA cables/adaptors etc. On the monitor, it says it's displaying 800圆00 when my Mac says it's displaying 1680x1050. 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).

The monitor's optimum resolution is 1680x1050 60Hz and RDM shows this as an option. 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.

Display adapter not showing up in resolution