Here is a test performed by Cummins on that boat with the twin 540s: https://dl.dropboxusercontent.com/u/.../Cummins1.html
Scroll down until you find the 43/45 numbers.
As you can see, fuel economy improves as rpm and speed drop, all the way to idle. So you have to be more specific. I don't think you want to run at 5 kts at idle even though you are getting 5 NM/gal..
Also you probably don't want to run lower than 1,200 rpm for any length of time to keep the engine loaded decently. See the tedious/nauseous thread "Under loading" for enlightenment on this issue.
Finally, those tests are somewhat optimistic as they are done with limited fluids, people and gear. You can make an approximate adjustment by taking your actual wot speed, ratio that to the Cummins wot speed and apply that factor to all of the data.