I´m an intermediate level windsurfer and using most of the time my 3S 107. I wonder if there is a default setup for the strap position? And what is the effect if i more the back strap more to the front?

FOR INTERMEDIATE SAILORS, I WOULD SUGGEST SETTING YOURSELF UP MORE FORWARD RATHER THAN BACK. THIS GIVES YOU A STABLE SET UP ON THE BOARD, MAKES IT EASIER TO GET INTO YOUR FOOTSTRAPS AND KEEPS YOU PLANING IF THE WIND IS NOT PERFECT. I WOULD START OFF WITH YOUR STRAPS SET ALL THE WAY FORWARD FOR BEGINNERS AND AS YOU GET BETTER AND MORE COMFORTABLE WITH YOUR BOARD, START MOVING THEM BACK- THE FURTHER BACK THEY ARE, THE MORE TOP END YOU WILL HAVE.