Going on past experiences of European tours & days of 300+miles I'd say 40+mpg is about right for steady 80mph ish cruising. I'd suggest you aim for 100 miles between stops but it'll do do 150 to a tank. A chance to stretch the legs etc. is the real benefit of sticking to 100 between stops. Best get some cycling shorts aswell
My tank ranges from bars to the seat. About 40 cm I believe.