I think one can get an accurate enough figure for the fuel flow, if one starts with a full tank, does two flights, identical in profile except one has a longer cruise period than the other, and fills up to the top (at a pump that is regularly checked by Weights & Measures) after each one.
From that one can work it out, because the climb/descend part will be constant.
Obviously this will be true only for a particular power setting and only for a particular aircraft. But this is how one calibrates the proper flowmeters: against a pump - I get mine within 1-2% and it gets re-checked at every fill-up. Much better than a dipstick.
I would also do this at the full-rich mixture setting; then anything one does with the mixture lever will be a bonus.
The range of a C150 is such that one must be really really careful, especially re the reserve for weather diversions.