It's in Ray Martin's book on page 111. You shoot towards the middle diamond on the end rail so the cue ball is about 6 inches from the head spot and on the line. If you are off 3/8 inch in your placement along the headstring, you will be off 1/8 inch at the object ball.... I have slept since using this but there is an easy way to make spot shots. I forget where the cue ball goes but you aim at the first diamond on the foot of the table. Ignore the ball on the spot, it is going in with no thought or calculation, no worries about what could go wrong. Somebody with a better memory can describe this shot. ..
I think it's easier to place the cue ball so that the shot is exactly a half-ball shot. You will be shooting with the cue stick over part of the corner pocket. Which part works for you depends on the table size and how sticky the balls are. This system is much less sensitive to ball placement errors.