Ok, I was walking about a VDP (visual descent point) which is generally associated with an non-precision approach. Its the point at which you must see the runway and start down towards it in order for the approach to be at a "normal rate of descent". HAT=height above touchdown on an approach, usually 400-800 AGL. Basically what you are determining is the point at which if you don't see the runway, you are going missed, even though you may not be at the MAP. Good example is a VOR approach where the VOR is the MAP but the VOR is located on the field. If you saw the runway just prior to reaching the VOR, could you still land using normal maneuvers and descent rates without losing sight of the runway? Can you think of many situations where you intercept a final approach course on a non-precision approach at 18,000AGL? I can't. The 6/3 rule is good for ENROUTE descent where a tenth of a mile one way or the other wont make a difference. The VDP point is more precise, usually to the a tenth of a mile (or a few seconds if you are using time). Example: HAT=650AGL will yeild a VDP at 2.16nm. Subtle but important difference.