You really want to add some rotation and/or twisting into your randomized agitation routine - it really helps with consistency and repeatability.Hmmm... Interesting, I'm not sure I ever used a twisting motion while inverting the tank, just a nice snappy inversion and back and a bump when setting the tank back down. In larger tanks when you invert, you can feel the bubble rising inside the tank and I would let it rise all the way before turning the tank upright again. I thought that was helping agitation, I guess...
It may be, of course, that there actually is something happening inside your tank that adds some rotational randomness there!