Lower 3rd is a general rule of thumb. I find the best practise is to keep your plant profile low through trellis nets and scrogging them (Screen of Green). I don't understand why people waste time and energy growing a plant 4 feet tall just to have to butcher the lower 3rd to half of the plant. Why did you waste the time and nutrients is what i ask myself? Why not trellis your plant and fill the net 3/4 with tops, then flip to 12/12, and tuck the first week, then allow everything to stretch up, then defoliate and watch your yield be epic, with little plant wasted.