Dear Pierre-Yves,
OK, now I understand. Regarding the drop in tower damping from 1% to 0.4%, this is discussed in the following forum topic: Natural frequency and damping ratio calculation - #12 by Jason.Jonkman. But basically, the 1% damping ratio is specified for the isolated cantilevered tower, but when the tower is coupled to the full system, the overall damping level will change.
Best regards,