I normally fill my Legacy's fuel tanks to about a fat finger below the opening and I have never noticed fuel venting.  The other day, I filled the right tank nearly to the brim and when I made a tight left turn on the ramp a friend noticed fuel venting from the wingtip.  I can't understand how you're venting fuel if the tank is only half full.  Is this while you're on the ground or in flight?

 

Dennis

Legacy, closing in on 500 hours tach time