There are a few people on here with a good knowledge of touring routes in Germany, I am sure Auntie Helen et al. with be along soon...
In the meantime I would recommend having a look at the Verlag Esterbauer website. They have some excellent guides to cycling in Europe (and beyond), while most of them are in German, there are some in English and even if you don't speak German, they are worth getting use for the maps...
Is there any reason why you have to start from Calais?
I must admit I have never cycled from Calais, but it looks very busy. We have used Dunkirk and found that after 20 mins or so nearly all the traffic from the ferry was gone, leaving an almost empty road to ride away on.
I have cycled from Calais to Germany and leaving Calais on small roads wasn't a problem. I did a route that went through the Ardennes, another bit of France, Luxembourg and popped out in Germany in Trier. I could specify it a bit more if you're interested.
It is really easy to leave Calais on small roads. On leaving the ferry and adter the French passport controls you will see a sign showing a slip road to the right marked "Centre Ville"
That brings you to a roundabout. Straight across and after a short while (about third of a mile) turn left for various small towns (le Fort Vert, Walsam) which takes youi on a humpback bridge over the motorway. Then Gravelines, Grande Synthe, Dunkirk, Zuydcoote, Bray Dunes, Belgian coastal route to Knokke (largely on sea front and dedicated paths), Breskens and ferry to Vlissingen, then any number of routes through Holland and along the Rhine.
The shortest way to Germany, starting from Calais is through Wallonia, the french speaking part of Belgium. RAVeL1 is the name of this track and it follows in Belgium a few canals and the rivers Lys, Scheldt/Escaut, Sambre and Meuse. Ravel1_Armentieres-Maastricht and http://ravel.wallonie.be. RAVeL1 is also a perfect way to skip all the hills in the southern part of Belgium!