It works, thank you 
It was probably due to the fact the I have my hdmi plug connected to a screen at the same time… 
If I remove the hdmi plug I can stay with Option "fbdev" "/dev/fb1" sorry about that.
So have to edit my first post to simplify it with the details that you give me.
Last question (I hope 
) more to know than a real necessity : how to find the best values for calibration ? I tried xinput_calibrator but the values are less good than something like Option "Calibration" "268 3880 227 3936"