Disclaimer: not a diesel mechanic
I had the same questions, and have found guidance that varies.
Load is more than just RPM. One major consideration is prop pitch. I believe that your engine's max intermittent rpm is 2500 rpm. ( low speed, high torque) The max continuous rpm is 2250. It's my understanding that if a prop is pitched correctly you should be able to achieve or come very close to design criteria at WOT. (including temps) If you can't make max rpm, then that could be an indication of prop pitch or other issues.
Your operating temp from what I found ( closed system) is between 168 and 197 degrees.
Your hull speed is somewhere around 8.8 kn. based on your waterline.
For cruising, I've seen numerous recommendations to operate at between 75-80 % of design criteria. However, there's also plenty of anecdotal information of diesel owners who regularly operate at lower percentages to conserve fuel with no loss of engine life.
My engine's max rpm is 3,000. I typically cruise at 1800 rpm (60%) which seems to be a sweet spot. (@ 180 degrees temp) With a clean bottom and smooth sea, that will produce 4.8- 5 kn. However, I'll run at 2,000 if bucking a current, and I've occasionally had it up to 2500 rpm for a few hours at a time to load it up. ( with throttle left over). My engine is also 30 years old.
At between 1000 and 1200 rpm, you'd be operating at between 40 and 48 %, providing your prop is pitched correctly. It seems to be running cool (155 deg.) at that rpm. (if the gauge is accurate). 1800 rpm @ 160 degrees is 72%.
I believe it's more important for its' health to run an engine at proper operating temperature. Once that's achieved, I don't think you hurt the engine running at lower RPM at temp and conserving fuel. You could do a series of runs at various speeds and fill the tank to record the difference in consumption.
See my disclaimer above.