Sort of a crapshoot to me. Since your chip or flop often goes over the elevation change it isn't as simple as just figured the numbers. I generally use what you are talking about as a guide but as I mentioned if you are flopping over a ridge for example then you have to take where the ball lands into account.
I also have a difficult time figuring out exactly what the elevation change on shots around the green means on WGT, it seems at times to be very inaccurate. I assume that it is from ball to hole.