PM's (power meters) for bicycles are getting less expensive. If you ever decide to get one, then you'll see first hand how much energy you save when you ride behind another. Even just one person in front of you is quite a bit of savings. Depending on the wind speed and direction you are riding to it, you might see more than a 30% savings of energy behind that one person based on my little experience since I got a PM a few years ago.

But as for your thinking that further back is better, it's very diminishing returns soon after that third person if you are in a single file pace line.