Graham Phantom damping fluid level.

Bob Graham says to use enough fluid to bring the level up to the top of the square profile machined on the bearing cap. The square is only about 1/4" in length. My question is, What level works best for you in relationship to this "square" profile?

I have tried only 3 different levels of fluid in my setup (maximum level to top of square, almost no fluid at all, & to the bottom of the square) but can't seem to obtain optimum results so I'm wondering what others use, although I guess it's really cartridge dependent. Is the fluid level super sensitive? FWIW I'm using a Koetsu RSP.