I find it's only owners with a bit of experience really know about power settings and fuel burn. This information comes with the same pilot flying the same aircraft on different trips in different conditions.
As most instructors don't, they tend to know very little about the settings and fuel burn in the actual aircraft they fly, usually a CFI will have told them something like "this one has four hours endurance".
It's just fill up and fly and if they have extra pax it's half tanks and fly, rarely for more than an hour anyway.
Tony