80€/h seems really low to me. I don’t think there’s a point of charging by the hour unless you’re doing very small features adds, tests, or maintenance work. I think one should always opt to get paid on a project basis.
If you have: excellent project management skills, can correctly estimate the required work, and have good clients who don't try to add "free" features half-way into the project, fixed-bidding 10-20% of the business value add can often work out better for you.
* Often that requires the client to have a good idea of how much they are willing to spend on the project, and how much additional revenue it's going to bring in (which is harder for smaller clients to do).
If any of the above is false, time-and-materials based pricing is often the better way to go in my experience -- it accounts for any "unknowns" in your estimation, and allows the client to add/change as many features as they want at any point in the project (it's easier to argue about a deadline getting pushed out than having to re-bid the project).