Kicker motor prop depth

I am going to add a kicker motor to my 169fs polar craft. My question is the kicker motor is a 9.9 Mercury 2 stroke. The prop  when the motor is put on the transom directly is about 8-10 inches plus depending on the weight in the boat. How much below the water surface does the motor prop need to be for optimal performance, or will I need to get a kicker plate? More money less bait...grrrr...lol. I know most bass fisherman seldom have a kicker motor but this also applies to trolling motors how far below the surface. Should that prop be?

sometimes I go over to the dark side and pickerel fish so the kicker motor helps a lot and is more economical than trolling with the 90 Yamaha and a trolling plate?

thanks in advance