Thanks for that ShyTorque
The likely reason your manual doesn't give any data on reducing fuel flow at altitude is just because the sea level data errs on the safe side and so the manufacturer didn't actually bother to produce any more data. More flight testing = more time and expense for the manufacturer.
Well it's Lycoming data, so I imagine the engine was bolted firmly to the ground.
Given only my post above, I can see why you'd think that might be the case. But I'm doubtful, because of the level of detail of the data provided. It would be surprising if they just didn't bother to measure the fuel flow, as they have clearly measured the power as a function of MP, RPM and altitude.
If our model is correct, shouldn't the fuel flow simply depend (neglecting temp for a moment) on the actual MP, which determines the charge, and the RPM, which determines the rate at which it is burnt.
So the two models seem to be:
1) Fuel flow is determined only by MP and RPM
2) Fuel flow is determined only by output power (BHP) and RPM
The procedure Lycoming shows for calculating fuel flow is considerably more complex than would be the case for model 1. It is clearly more conservative. I could understand that if the truth were somewhere between the two models, but not if it were much closer to model 1 as we believe.