Nepoužívám, podle návodu to nefunguje?

Changing the rulers' unit of measure
In the default template, the unit of measure used by the rulers is px (“SVG user unit”, in Inkscape it's equal to 0.8pt or 1/90 of the inch). This is also the unit used in displaying coordinates at the lower-left corner and preselected in all units menus. (You can always hover your mouse over a ruler to see the tooltip with the units it uses.) To change this, open Document Properties (Shift+Ctrl+D) and change the Default units on the Page tab.