How to get 1680X1050 resolution for an external monitor?

Hi all,

I have an external monitor I'm connecting to my new Macbook Pro. It needs to run at 1680X1050 resolution, but in the system display settings this isn't available.

After googling, many suggestions are to simply hold down the options key while pressing 'scaled' under the resolution, and while this does show many more resolutions, as shown in the screenshot, the exact one I need 1680X1050 isn't there.

Does anyone know how I can get this resolution to work?

Thanks