Europe can be difficult to compare. My personal ranking in salaries is this: UK > Scandinavia > Netherlands > Germany > Spain > Eastern Europe. German salaries are imho not very good in the digital entertainment industry. I don't know about France, but they may be similar, if not higher. The biggest problem is that social benefits, services and holidays can be quite different in terms of quality across countries. Also it makes a big difference if you live in a capital. E.g. prices drop a lot once you leave London or Paris. Germany is a bit unique though. Berlin is cheap while Frankfurt and Munich aren't.
I would suggest asking on expat forums for IT developer salaries. That should give you a range. Reddit may be a place to ask. Might also ask for prices for food, eating out etc.
I'm pretty sure there are tax calculators for France, so you can figure out the tax. If possible do some search for apartments in the area you're targeting. In general, when living in Europe and holding a decent job your rent should not put a major dent in your budget, unless you live in one of the expensive capitals, like Paris or London.