Depends on what "ideal" is your priority. Fastest cruise speed, most economical fuel use, or maybe a little of both?
A couple things you will need is an accurate tachometer and way to measure fuel use.
A digital photo tach is less than $50 new and should be a standard tool for anyone with an older, non-electronic diesel. Make a simple table and record photo tach verses dashboard tach readings. The dash tach is usually driven by pulses from the alternator. 10 percent error is not uncommon.
Fuel use is harder to determine. There are plenty of threads on figuring that out.
1984 Monk 36 Hull #46
Currently in New Jersey.