My rule of thumb is melatonin subtracts an hour. That is: if one slept for 7 hours, one awakes as refreshed as if one had slept for 8 hours (and so on).

That's the opposite of my impression. I used melatonin for several months because it made it so easy to get to sleep. But I found that I needed to get a full 8 hours of sleep to wake up refreshed. I attributed that to some vague notion that the melatonin needed that much time to work its way through my system. At any rate, waking up after less than 8 hours seemed harder than it does without melatonin. That was my very unscientific impression.

For that reason, I was glad when I found a mental trick that let me get to sleep as easily as I could with melatonin.