Head East as soon as you can.
The shortest route for Calais - Chatel is obviously via the middle of France (Reims, Troyes, Dijon, etc...) but that's rubbishly flat and straight. To enjoy your trip you want to avoid that. Head towards Metz, and then the Vosges, Alsace -route des Cretes- (my lands
), Jura, Savoie and there you go. Much much longer, but infinitely better.