Yup, I confirm there is no 10% leeway in France. I have lost 3 points in 18mths for doing exactly that kind of over the limit. When I change my car I'm going for one with a digital speedo so I can see clearly when over, I now try to set my limiter, especially on motorways.
Catharine was just (and I mean in the last ten minutes) fined €80 for doing 56 in a 50 limit, she is livid. There is a ten percent allowance for speedometer inaccuracies which effectively means she was 1km per hour out of luck.