In the meantime (a digression, not meant to hijack...), I've gained almost a knot at various RPMs simply by changing to (accurate) digital tachs.
The previous analog tachs were "on" at about 2000 RPMs, but not so much apparently at idle, low speeds, high cruise, and near WOT. Nor was there a way to adjust them so they'd be accurate across the full range.
So now I see that what I thought was 6 knots at 800 RPMs is actually 6 knots at 725 RPMs. And what I thought was 20 knots at 2300 RPMs is really 20 knots at 2175 RPMs.
I haven't had time to fully capture and chart the new figures, but in the past I spreadsheeted two-way calm-water speed readings at various RPMs, trimmed each run to the highest speed possible at those given RPMs, and then used the engines' nominal burn rates (from published data) to calculate nominal NMPGs.
Maybe the digital tachs have already saved me enough money to afford proper flow meters.
Or maybe the nominal numbers are close enough so I don't have to prioritize how to run more wires from engine room to flying bridge.
-Chris