If you have a regular highway commute, you might consider resetting your trip computer before that trip. Drive the trip at 50 one or two days (road conditions permitting). Then reset the trip again, and drive at 60 a couple of days. Then reset it again, and try 70.
This method will make it more difficult for you to see your tank mileage (during the duration of your testing), but it will give you a better idea of how your Prius handles the freeway at different speeds.
It also negates the necessity (and inherent inaccuracies) of having you constantly look at the MFD while driving. Just drive at your chosen speed, and record your mileage when the trip is done.
Consider doing your test with "round trips." Sometimes the trip in one direction or the other is mostly uphill, downhill, or into/with the wind. Recording your one-way direction may be misleading. After all, you want to know what's the best overall highway speed, and your Prius is not equipped with a windspeed indicator, so you cannot make mental or mathematical adjustments for wind.
On my car, I believe the best highway speed (for MPG) will be approximately 50-55 MPH. I was driving 48 yesterday on a local street, and finally got one green light (so I didn't have to stop). At a steady 48 MPH on a relatively flat grade, my MFD was consistantly showing between 68 and 98 MPG. Even the worst mileage I was getting on that mile-stretch was better than the EPA estimates for the car!
I do plan to experiment more scientifically, but probably not until after a few thousand miles on the ODO. I'd rather let the car get broken in really well first, so I don't have to do the tests over.