Thanks for the fast replies !

I'm still have not measured the prebend, but I will, and give it. (Today I'm far from my boat)

Any case, I think the prebend depend on two factor, the spreader rake and the diamond tension. (Is it true ?)

The spreader rake recommendation by Nacra is 35-60mm, mine is 35mm, really the minimum.

The diamond tension recommendation is 34-40, mine is 39. It is maybe too much... (But I was afraid broken my mast)