Missing info on taxation to evaluate job offer

Good morning everybody.
I'm originally from Italy and currently expat in the Netherlands.   
As I'm evaluating an important job offer in France, I would like to ask your support to clarify on taxation. 

I ancitipate the question here for whom would not like to read the entire long post: Can anybody confirm if the income tax has to be calculated on total Salary already net from social security tax or both social security and income tax are to be calculated on full gross salary?

This should be quite a simple info for who's already living in France, but for some reason, despite all my readings, and even asking some people i know in France, I couldn't manage to get clarity. I really think that experts in this forum can help me giving a final answer.

In evaluating the job offer in Paris, the head hunter has given me an idea of net salary that in my opinion, after all my reading and calculation, is wrong (too low). I would really avoid to lose an important job offer because of inaccuracy on the info.

So, I understood in France there are (cut sharp) 2 big taxes to pay: 1 is social security (which I understood is approx 25%, that is already taken by employer in payslip) and one is Income tax (which is paid via a tax return and is built in tiers like many countries).

The key info I'm missing is: The income tax is to be paid on the total gross salary or on the salary already net from social security (e.g. the net salaries in the payslip?

From all my research, including specialized sites like KPMG and French-property, I really got the impression that the income tax is to be paid after deducting social security.

A simple example:
Assume a salary of 50.000

- Deducting social security (25%) it would get to 37.500. This is the base for income tax
- The due income tax, based on 2016 tax rates (in the ranges of 0%, 14% and 30%) would be 5.600
- The remaining is the net salary, so 31.9, which divided 12 months equals approx 3.200.

Is this correct and complete with no other tax due after this? (except habitacion, TV etc..that are fine)

Because the alternative to this is that both social security and income taxes are all based on the full gross 50.000 and this of course would make the taxes much higher.

I also heard that there is a deductible 10% that is automatically applied to salaried employess, but this is not the core of my request.

So as per my original question: Can anybody confirm if the inncome tax has to be paid on Salary already netted from social security tax or both social security and income tax are to be paid on full gross salary?

This is really the key missing info I need to properly evaluate the job offer.
Thank you so much in advance for your help!
D.