4k display: Limited scaled resolutions when in portrait?

Hi,

I've got a MacBook Pro, late 2013, retina, discrete graphics, Yosemite. I like it. Also have a Dell P2815Q monitor. I hate it. I would, however, like to gain better control over the resolution of the Dell display when in portrait mode. Currently, the monitor will run in landscape at 1504x846, 1920x1080, 2560x1440, 3008x1692 and 1:1 at 3840x2160. But the system display dialog will only allow four options when the display is rotated 90°, 540x960, 846x1504, 1080x1920, and 2160x3840.

I would like 1692x3008 or 1440x2560 while in portrait.

I have tried SwitchResX. I entered a custom scaled resolution and tried a few other things and it did not work out well (I ended up deleting plist files to get things back to normal).

Any help?

Thanks