nyctc7: Is that right or where am I going wrong?
Doing that division is where you're going wrong. Forget the math. All of that has already been figured by you just knowing how many moves it would take to get 15 feet on a VF green. You already calibrated that by figuring out your putter's moves on Very Fast greens.
Here's how I look at that 15 foot putt. My Ghost, on VF greens, goes 7-8 feet on the 15 scale for 1 move. It goes 12-13 feet for one move on the 25 foot scale. It goes 13-14 for 1 move on the 50 foot scale and 18 on the 100 foot scale.
Now, I can hit 15 feet 4 different ways. 2 moves + on the first scale, 1 strong move on the 2nd scale, 1 strong move on the 3rd scale or cancel the 1st move on the 4th scale. While none of the moves gives you exactly 15 feet, the situation of the putt determines which one you use. If the putt is flat, cancelling the 1st move on the 100 scale would get you about 16 feet which is a good speed to hit that flat putt. VF greens respond well to aggressive putts so I would probably use the 100 scale and cancel the first move.
Take the same putt on Tournament speed greens. The 4 values are going to be different now. In my case, 8, 13, 15 and 18 are what I use for the moves on those 4 scales. Notice how little difference there is between the 2 green speeds and the values I use. That's what I have learned with my putter. A 15 foot putt on Tournament greens is 1 move on the 50 scale for me.
Which scale you decide to use is up to you. I tend to use the higher scales for the shorter putts because of the short backswing and ease of dinging.