Moving the motor back will not change where your cg needs to be relative to the leading edge of the wing. But moving the motor back sure wakes up the 3d characteristics of the Extra Stiffy. I am pretty sure my cg is around the 5" mark. The best way to check your cg is trim for level flight at about 3/4 power and then roll inverted. If the plane dives you are nose heavy. If the nose climbs you are tail heavy.