120 Lehmans should run 2500rpm at WOT in gear.
Anything else is under propping or over propping.
One usually uses pitch, diameter and trimming blade area to achieve rated rpm in gear.
I recently mounted a new prop and am down 100rpm. So I'm 100 rpm over propped. The boat has less power, runs to fast at idle and the engine seems to not run as freely as it did before.
I think American Marine is telling people what they think they want to hear. 2250rpm is well over propped and telling people to "cruise" at 1800 is excluding many very important variables. From all I've heard about AM I think one should seek advice elsewhere. Boat Diesel.com is a good place to look but of course there will be opinions there too. I've said for some time here to seek technical advice from engineers and manufacturers and seek service from mechanics.