How to properly scale qgroupbox title with stylesheet for high resolution display?

Question

I'm trying to apply a stylesheet for a QGroupBox (qt 4.8), which works on normal screens (96dpi) as well as on high-resolution screens (yoga 2 pro, 3200x1800, 275dpi, windows 8.1).

The following style looks good on the 275dpi screen, but on a regular 96dpi screen the top margin is by far too huge.

QGroupBox
{
    border: 1px solid red;
    border-radius: 7px;
    margin-top: 12ex;
}

QGroupBox::title
{
    subcontrol-origin: margin;
    subcontrol-position: top left;
    padding-left: 10px;
    padding-right: 10px;
}

Changing the top-margin has an effect, but I can't find a proper setting that works on both screens. If I make the value smaller, the content of the groupbox overlaps with the title on the 275dpi screen. I was also playing with the units "ex", "px", "mx", "pt". Reading the documentation I would have guessed, that "2ex" was the correct solution, which should scale with the different screen resolution.

Without stylesheet, the groupbox works well on both screens.

How do I properly design my stylesheet?