Thank you all for your answers
It's not to a certain board or sail. I just want to know in particular.
What I can read here is that you almost have to try a fin to know it's good (or know a person that has tried it), is that correct ? Doesn't seem to be any rights and wrong as long as you're in the boards recommended scale. Just a matter of former experiences and what you feel is good for you. Am I way out here ?