BIK. Good points - and thanks. Can't quite trace where I read it, but I believe one purpose of the fuel flow gauge is to set the fuel flow needles at the density altitude for takeoff indicated in small figures at the edge of the gauge. I presume this means that for take off at say 8000 ft DA (typical PNG highlands on a hot day) you go to full throttle them lean until the fuel flow needle is opposite the DA figure. This then gives you correct take off power for the ambient conditions. Obviously if you failed to lean under these conditions the power loss is significant.
If that is the reason for the various altitude figures listed on the take off area of the fuel flow gauge, then that would suggest that the fuel flow gauge is certified to a certain accuracy. If it is found not to be accurate, then presumably the defect would be picked up by an alert LAME during the 100 hourly who would, if diligent, re-calibrate the fuel flow gauge. Normally one leans to an EGT figure for cruise, and to fuel flow when setting the mixtures for take off at high airport density altitudes?