Bruno, it certainly isn't "dumb" to have trouble finding a procedure in a Denon manual, since they're experts at concealing these things. It is, however, unrealistic to think that the Denon microphone and associated circuitry are less accurate than those found in a Radio Shack or other inexpensive SPL meter, and that added human error in reading the meter doesn't make it even more likely that the manual calibration is the less accurate of the two.
Be that as it may, I took a quick look at the 890 manual and see that under the Settings section the Manual Setup for Channel Level is described at p.29. These pink noise test tones are at 75dB.
