Never use a rule

I seldom use a rule for marking out. I always try to mark the length of something straight from where I want it to fit.
In this case I'm marking the width of the box onto a tray piece. I'll then set a stop on the saw and cut a test piece before cutting the real thing. At this point I'm looking for a tight fit - the dovetails are then cut about half a millimetre over size and the tray then fits into the box perfectly.