What rev range

[Mac3]

Hi all I have just repowered my 22 ft f/g cruiser with a brand new Suzuki 150 4 stroke so I've kind of migrated from another iboats forum. I guess I could ask the dealer this question but I look forward to real life replies. Please excuse the bad language i.e metric but it is irrelevant to the outcome. So my engine in flat calm conditions at 4000 revs does 20 knots and uses 1 litre of fuel per nautical mile. At 5000 revs it does 30 knots and still uses 1 litre per nautical mile. At the 1st service the mechanic noted that I'd spent most of my time around 4k. He didn't say whether it was good or bad and I didn't ask. I guess he was trying to impress me that he could get inside the brains. So at 30 knots you get there quicker and it costs the same. What is the best for the engine?