The information I have is that you are use the first track flare ring as a reference.  You place the mb half way between that and the vertical axis line.  I posted this after realizing that I had drilled a ball for myself (Shock Trauma) with the mb half way between the grip line and the val.  I remember drilling the ball quickly and made the mistake of placing the mb there instead of in the strong position, where I intended it to be.  I really don't think it made a difference, the ball is just as strong and angular at the break point as my other stuff with the mb under the thumb.