Once you visualize it, it will snap into place. You said that you want to use 9 points. That's a good number. When AF is working properly, those 9 points will form a nice v-shaped curve (you have probably seen the curves posted here). The "wings" of the "V" will have four points each, and the last point will be at the vertex of the "V" is everything works well. That's what a good AF curve should look like.
To set your step size, you want the tips of the "V" to have a HFR around 3 times the HFR at perfect focus. Now, remember that the 9 points are spread around the "V" with four on each side and one in the middle. That means that the tip of the "V" is 4 points away from perfect focus.
So you were on the right track. Get good focus and note the HFR and focuser position. Then move focus until the HFR is about 3 times bigger and note the focuser position, just as you described above. But, instead of dividing the difference in focus position by 9, you should divide by four. That should be your step size.
Tim