HiDPI not work perfect on new x1c

hi, I have 2560x1440 x1 carbon (2014) installed el capitan these days.
I made the original pixel works which makes text extremely small
so i want to enable high dpi model

By using SwitchResX, i cannot enable 1280x720 hiDPI, but only 1280x800.
the later one(1280x800) makes the screen not fit well (left and right side empty, bottom of the screen flicker):

by the way, the...

HiDPI not work perfect on new x1c