Thanks for your analysis. I've found a document written by Claude LELAIE about light aircrafts spin. It is said that every French light aircraft are certified with a maximum imbalance of 25% if wing fuel tank equipped. So, it may be only a French rule. As I'm flying a TB10, I've sent a request to SOCATA to get their flight test conditions. Do they have a regulation about that, or just a common way of doing that...
I'll keep you informed of their answer!