Genrege
I think that you need to go back and read the book because I think you have misunderstood it, from memory ( my DR400 flight manual is at the other end of Europe) at an altitude of 8000 ft the engine makes 75% power at 2700 rpm. The 75% rpm at sea level is about 2550 rpm. So anything above 2550 at sea level and you should not lean the mixture.
This is the reason that I have drawn the graph and keep it in the aircraft because it is the only accurate way of making the speed and fuel consumption numbers.
I have a house in France just north of the Spanish border and can only make it non stop in the DR400-180 from Oxfordshire if I get all the numbers set correctly.