Peter:
User reported fuel consumption data is inaccurate for several reasons: Flowscan or similar meter inaccuracies as you note (does anyone rigorously calibrate their Flowscan?), Generator and docking time erroneously included in travel time vs fuel fill data, and probably most importantly- owner wishful thinking.
Your rule of thumb broadly works. I proposed a more comprehensive one that takes into consideration displacement, hull type and engine type in a related thread. See
Trawler Forum - View Single Post - What is your mpg, gallons per hour etc... at various speeds?
By making the rule of thumb based on power to reach displacement speed, you automatically consider water line length. A longer water line leads to a higher displacement speed and better NM/gal. If you have two boats with the same displacement and hull type, the one with a longer water line length will achieve better fuel mileage.
But maybe the biggest variable is hull type. True displacement hulls are fairly consistent in hp per thousand pounds to reach displacement speed. The data shown in Bebe's book (edited by Leishman) shows a rather small variation of maybe 20%.
But semi-displacement hulls vary all over the map at low speeds. One poster on the referenced thread claimed he used twice the displacement hp or 3 hp per 1,000 lbs for a semi-displacement hull. My downeaster style Mainship Pilot 34 is notoriously inefficient at low and high speeds.
A less significant variable is engine type. No matter whether it is a 30 year old 5.8 liter 120 hp Lehman making 40 hp or a new 380 hp Cummins QSB making the same hp, they both make about 16 hp per gph of fuel. Only a modern smaller displacement engine like the 4.5 liter JD will do better because it has less iron to turn over and its fuel injection is optimized for the lower hp output.
So, we have seen this and another thread where members were asked what their fuel consumption numbers were, and lots replied. I take all of this data with a big grain of salt and prefer to use the foregoing to estimate low speed fuel consumption.
David