Complex Shipping Rule: €3 Flat Rate for order's first 2kilos + extra €1.2/extra kilo charge
One of the problems with Commerce (and Ubercart for that matter) is that it takes awhile for someone to come around with weight based calculations. I have done something similar but it might not work for you.

1) I made each increment I wanted to allow a single product (i.e. 1lb of Product X is 1 QTY, if they want 2lbs of Product X, it's the same as saying Product X with 2 QTY.)
2) Then I created bundles for what I wanted to allow.
3) In shipping, you can set up a rule that allows extra for each extra QTY ($6 for 1 QTY and $2 for each additional QTY.)

For you, it might be possible to set your rule to multiply but you would have to create a module that would allow you to set percentages on QTY if you wanted someone to be able to add 1.2 QTY to their cart. For me, it was easier to do by whole increments and much cleaner on the rules. You can also set your shipping rule to line items so that the first line item is the base and the remaining are the additional.