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I assume you inform your students about this 'policy'. If I was one of your students, I'd book the plane for 1 1/2 hours and you for the 1 hr to get you in the right seat for the time I need you. That way, you could overlap a bit and you could jump out of one plane and into another and therefore avoid the 'hi and bye' time.
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...of course I do. And as far as your 'overlap' scheme, my only hope is that when you become a CFI someday that you have four students in a row who do that to you consecutively.
In 14 months of being a CFI, I have NEVER had anyone suggest I was overbilling my time. I HAVE had people correct me on more than one occasion when I said, "ahh.. we BS-ed for a while, that'll only be 1 hour of ground." and they reply with, "no, I have been with you for two hours, so its two hours."