Yes, GG, the Floscan will tell you instantaneous fuel consumptions rates. See Floscan Instrument Co. Inc.
You would extend that basic data with environmental context: speed at a given RPM level, modified by effects of tide/current and wind
to the extent you can try to do that within the period of the sea trial. (A given RPM setting can render different speeds if you're heading into the tide/wind versus running with tide/wind.) A more accurate picture would present itself over time, where you'd be able to do some averaging... but probably the figures captured during sea trial will be useful enough.
You'll hear most diesels like to do most of their work within a specific RPM range. Various manufacturers -- and their individual products -- can be slightly different, but generally guidance shakes out to run at 75% of wide open throttle (WOT), 80% of WOT, 85% of WOT, or even "200 off the top" (i.e., 200 RPMs below WOT). Essentially, the motor
should be operating at the right temperature, and these percentages are often easy ways to know you're doing that.
You determine WOT in two ways. One is to get the manufacturer's operators manual (often available on line, or in hardcopy from the current
owner during sea trial) and read the rating (e.g., 300 HP at 2500 RPMs, or whatever). The second method is during sea trial; make sure the engine will achieve that rated WOT -- 2500 RPMs, in this example. (If it doesn't... long discussion about boat weight, bottom condition, tankage, props, etc. side-stepped here.... but it's not the end of the world.)
So you take GPH readings from the Floscan at those various RPM settings (WOT and varous percentages of WOT) and you take speed over ground (SOG) readings from either the Floscan (if so networked) or from an onboard GPS
All that said... The engine manufacturer will also have "performance curves" available. These are sorta generic, but they will also show nominal fuel consumption under load at given RPMs. That doesn't quite give you speed info, but if you look at a fuel flow curve, you usually see a big increase as RPMs increase. (Duh!) The Floscan will tell you similar info; GPH increases but in a displacement hull boat speed will NOT increase at the same rate. The manufacturer's nominal info will get you in the ballpark, and the Floscan readings will show you how nominal translates to "in that boat, with given load, in specific sea states, at specific RPMs" and so forth.
So in your sea trial, as you approach hull speed (compute that: 1.34 times the square root of the boat AT THEWATERLINE (not overall length)) you'll see RPMs increasing, fuel consumption increase, speed... not so much. (Example computation: imagine a 70 foot OAL boat with guess-timate 60' length at the waterline (LWL); square root of that * 1.34 renders approx. theoretical hull speed of 10.38 kts. [Note: That may be why your broker is telling you info in the 10 Knot range.] More horsepower above that hull speed spends fuel with little speed return. Although you can probably fly a building, given enough horsepower... usually theoretical hull speed for a displacement hull gives you a good idea of a reasonable maximum target.)
Comparison of fuel flow (from Floscan reading) at that hull speed gives you what you're after, which is essentially NMPG at a given RPM setting/travel speed. And the rest of your notes can help you work out various NMPGs to see whether running at less than hull speed is sometimes more economical ENOUGH to be worth the loss in speed.
Pardon if not clear; I just typed it off the cuff... so I maybe didn't describe the simplest way to understand... Ask if you need clarification.