The manual that came with my minn kota edge 45 said this:
ADJUSTING THE DEPTH OF THE MOTOR The propeller tip must be submerged at least 12” to avoid churning or agitation of surface water. 1. With the motor deployed, firmly grasp the outer shaft or control head and hold it steady. 2. Loosen the depth adjustment knob until the shaft slides freely. 3. Raise or lower the motor to the desired depth. 4. Turn the motor control head to the desired position. 5. Tighten depth adjustment knob to secure the motor in place.
So as long as you are deeper than 12" it is up to you how much deeper you go. I would try different depths to see which runs the best and quietest.