The normal way might NOT be the correct way (in the eyes of CASA).
In the POH it says to lean the mixtures by reference to the EGT and so I believe this the correct way to do it.
Use fuel flow only as a means to crosscheck the mixture set by EGT.
However with accurate instruments (EGT, Fuel Flow) and accurately set MP and RPM, the leaned mixture will always be at the same EGT and same fuel flow (eg. 1450 EGT, 72litres/hour) on a particular aeroplane. I suppose there would be nothing wrong in this case once you know the particular aeroplane well to lean the mixture using Fuel flow and then crossheck the EGT.
Just be careful as the peak EGT varies from aeroplane to aeroplane ( I have seen ranges from 1450 to 1625).