I work in metric system, centimeters instead inches.
So I use 'dpcm' and 'ppcm' instead of 'dpi' and 'ppi': so I can set up 120ppcm which equals 304.8ppi and have perfect linear sizes for my designs.
But Krita doesn't allow me neither to set 'ppcm', neither to set fractional resolution.
Please, add this feature.

The scale image dialog allows you to select pixels per centimeter in the resolution setting, but the new image dialog is hard-coded to ppi. Please add wish report at bugs.kde.org for krita. Implementing the feature will have to wait until after the 2.9 release, though.