yea, i'd agree that there isn't much of a useful rule for that as you probably well know it varies with speed and direction constantly throught the day and night.
but what's given in the Met syllabus as a "rule of thumb" for this is that from 2000ft AGL to the surface the wind backs by around 30 degrees and speed drops to about 50%. ...only applies overland though dut to friction layer...is a seperate one for over sea.