Thanks to Mr Garmin, here are some proper nautical distances:
Cherbourg to Le Havre Marina (shortest distance) 70nm
Cherbourg to Le Havre Marine (coast hugging) 90nm
La Havre Marina to Rouen 70nm
Rouen to Paris 130nm
I don't have hi-res chart data for the last leg of the Seine, so have erred on the long.
With Jersey to Cherbourg via Alderney also being about 70 I can see a pattern forming! 5 legs of 70nm, not allowing for river current.
Day 1) Jer - LHM, 150nm @ 30kn, 5h with stop in Cherb for fuels
Day 2) LHM - Rouen, 70nm @ 15kn, 5h
Day 3) Rouen - ?, 70nm @ 10kn, 7h
Day 4) ? - Paris, 70nm @ 10kn, 7h
Day 5) In Paris
To do it in 8 days in all that leaves 3 to get back from Paris to Jersey. Might be possible to ride the current back between Paris and Rouen in one day?
Edit: Sorry, forgot the point, with a total trip distance of around 700nm the fuel used is going to range from 500ltrs to 2000ltrs.