I Believe in using a hinge about every 4 inches apart and two hinges close together at the tip near the end of the flap.
My reasoning for this is that under load when maneuvering, especially in hard corners, the flap can flex a small amount between the hinges and actually slightly stiffen the movement at the hinge and cause slight stiffening of the controls at a critical time. It's only a theory of course but a sound one, I believe. Also with less load on each hinge there will be less chance of any hinge failure.
Two are used together at the tip because that is the most likely place for destructive flutter to develop if the flap is not very secure!
Randy Cuberly