Getting grooves of equal width How can I get a groove of uniform width, both horizontally and vertically, on a door (a plane)?

I've used edge loops, subdividing the surface and the knife tool but I can't get the groove to be a consistent thickness (6mm).


 

One value is about 736 and other is 730, which is close but not exact.
I would use the bevel tool.

In edit mode, place one loop cut where you want each groove.  Then select the edge loops and hit Ctrl+B and drag to what ever width you want the groove.  The bevel tool splits each selected edge into two edges connected with a face.