Adding/Using HiDPI custom resolutions

I've tried to do this quite a few times. My 4540s has a 15" 1920x1080p monitor. Most of the time, I can work close enough to the display such that the text is not too small. But sometimes, it would be useful to work at 1600x900. Unfortunately, if you use a normal scaled resolution at 1600x900 the text is not as crisp. It would be great to use Apple's HiDPI tech to get better scaling.

Same thing goes for my Lenovo u430 with 1600x900 14" display. Sometimes it would be great to use...

Adding/Using HiDPI custom resolutions