for me the nice spot for the D3C is between 1280 and 1340 rpm about 4 lt. 1420 rpm is also a sweet spot. Propeller is Autoprop H3. Before longer Motorpassages, I dive and clean the Prop.
A couple of years ago I made a nonstop 100h Motorpassage with rpm 1640 -1720. I would not recommend it, because the whole Engine room gets pretty hot. Even the watermaker did not work anymore, because the fuses were too hot. Since the fuses in the Engine room are thermal fuses, you might get trouble with other equipment because of the hot fuses...
Also important for me is a balance between speed, noise and vibration. Over 1600 rpm it's getting noisy. Over 1800 it's getting very noisy AND a lot of Vibrations. When I go over 1800 rpm it feels for me that the boat is nearly falling apart (-: Strong Vibrations.
You will get a good feeling, what you and your boat "likes". At some rpm, the water level stick is producing vibration noise, At some rpm, the oven produces vibration noise and so on....
I think every boat has a slightly different "nice spot" without any annoying vibration or noise. My sweet spots are 1280 - 1340 and 1420. I rarely cruise under 1200. There is also one rpm area, I think it's around 1000 rpm, where my D3 does not hold the rpm. Thats the area where I think the turbo is Opening, but when opening the rpm goes up and the electronis are closing again which results in permanent rpm wandering up and down.
The Idle rpm is about 700-750.
Maybe someone has the same: When running Idle, my 24V Mastervolt alternator is not charging. It starts charging at about 800-850 rpm. Any Idea how to get it charging at Idle? My Volvo technicians had no Idea.