Taxation is complex in France , I guess you understood that by now .But every Country has its own surprises , it is called CSG and CRDS in France ( 9-10% extra social contributions) or Kirchensteuer in Germany ( 8-9% extra ) etc...this in addition to standard income tax.
In addition in France if your property and portfolio is valued to exceed a certain amount you will be subject to ISF ( Fortune tax) etc..
Best advice I would give you is next time you are in France , visit a tax consultant with your contract and your intentions and he/she will get you a good estimate of what you will take home in the end.
If you speak good French , you can also visit a governmental " centre d'impot" to get the same estimate for free.