Anybody intelligent enough to pass the PPL exams can be taught how to lean the engine to set up a specific operating point e.g. 65% power, peak EGT.
Or (if not paying for the fuel) just fly full rich everywhere but accept a range reduced to around 60-70% of what it could be.
Then do the two flights to establish cruise fuel flow.
Then you can do proper fuel planning.
Everything else is guessing, which is OK in the school scenario where one never goes anywhere far.