The units are in drawing units (which depends on the viewbox.) If the scale is 1000 (in the latest Inkscape, 10.41667 in earlier versions) the units will be 1/1000 of an inch. Otherwise they are proportional to viewbox size. The easiest solution is to set the tool bar to in or mm remembering that the start is the top left of the drawing element, so the x and y spacing should be exactly 0.1in in the tool bar display. An example using Inkscape 1.1.2 (where scale is 1000)
First via the tool bar set to in:
the next pad is 0.1in down in y
The scale is almost 1000 (fp roundoff in action!)
so the coords in xml editor are in 1000th of an inch
again modulo floating point roundoff issues, cy goes from 277.96075 to 377.96075 (exactly .1 in in this case) referenced to the top left corner of the drawing element (not the center of the circle!)
edit: In turn the start is the top left of the view box as 0 0.