Testing KRuler with "QT_SCALE_FACTOR=1.7" (i.e. fractional scaling like on a HiDPI display) works fine in general, but shows some oddities:

1. All measurements are scaled up too, meaning the value displayed does not relate to the actual number of pixels on the screen. However, I'd argue that first and foremost users are interested in the pixel dimensions of something displayed on the screen e.g. developers measuring sizes before and after when adding HiDPI support to applications (: or web designers working with images displayed at 100% zoom (i.e. a 1:1 mapping of image pixels to screen pixels without scaling). This means KRuler should not scale the measurement.

Still, sometimes you might be interested in the virtual/scaled size, e.g. a developer measuring a default window size (which would be scaled by Qt depending on scaling) or a web designer working on a layout. This could be added as an option (either replacing the other value, or as an additional measurement).

2. For fractional (non-integer) scaling factors, the tick marks on the ruler look really bad, because they do not align with the pixel grid anymore. They should not be scaled. (One more reason to default to a non-scaled measurement).