Yes, as folks have said, the site is giving you "per engine" guesses.
Not so easy to pin down the "sweet spot" or even a best "cruising speed" because that depends on hull
form, weight, sea states, etc. That site's info can be useful, but I don't see where it can give you speed (therefore NMPG), comfort level (relative to sea states), and so forth. 3000 RPM
? Maybe sweet, maybe not. Cruising speed 22 knots? Maybe at that RPM, maybe not (and I'd guess not, if that engine revs to 4800 RPM).
FWIW, with diesels, the common rule
of thumb for efficiency is often stated as 75% of WOT, 80% of WOT, 90% of WPT, "200 off the top" -- IOW, the rule
of thumb is slightly all over the map -- because there are other issues involved relative to RPMs and load.
You can often find the manufacturer's nominal performance curves for a given diesel