I think it's all a bit relative?
Work out what yours is at the moment using fuelly then take a chunk off to account for luggage, extra person, strange roads won't be as smooth etc.
+1 on the Sunday, and don't get on the ferry nearly dry, first petrol station on the motorway from Calais is a long way... I turned off with an empty tank and managed to find somewhere, wasn't fun though!
I thought I'd read in MCN that Yamaha claim our bikes are ok with E10? They said anything after 2000 or 1990, can't remember, but I know my 04 plate is safe