Fuel flow is the better way but one needs to have an accurate FF meter to do it that way. Outcome is pretty much the same using power setting though.
When you say "power setting", does that mean RPM for a fixed pitch? Or throttle position? Or some combination?
While it doesn't really matter if power is proportional to fuel flow (as long as it's a monotonic function), you do have to have a way of knowing that you have the same power, more power or less power at different times (i.e. when you're trying out different speeds). I would worry that issues like propeller efficiency differences with speed make that difficult to assess. The differences may be small, but the differences in power required with speed are actually also quite small around best endurance speed.
But I haven't done the experiment and you may well be right that FF is not practically required.