You miss my point Sloth and unfortunately don't follow--accuracy & precision is not the aim, but rather a simple arithmetic process demonstrated to estimate gas consumption that you can do both pre-dive as well as on-the-fly, in your head, and after some practice --without whipping out your wetnotes anymore for reference.
You are good at math, I suck at math.

On the fly can get some people into trouble. Id rather do all the heavy lifting before the dive instead of during.