Despite reading about pulse and glide and all sorts of tricks, I've finally cracked it. The secret is (gasp) slow acceleration. There is an occasional sweet spot that comes up, where you can be travelling at say 50mph, uphill, accelerating very slowly, and getting about 50mpg. Once you've got to cruising speed you can do much better than mpg. Finding the sweet spot isn't easy though.
So my latest mpg calculation for my most recent tank is 60.03 mpg (imperial) 49.99 mpg (US).
For my next trip, I will pay some attention to the road. The new Prius has a heads up display which would make life safer.