I put mine in the top skirt of the keg, elbow it down and elbow it again to create a whirlpool the actual output is at the 3 gallon mark. Reason being is I use the cam lock fittings, and was too cheap to get another ball valve to close the port when the quick connect was disconnected. Its also nice because I recirculate my mash so the recirculation port is at the same height, no slack in the hose.
I have mine at the 5g mark that way it is just below the surface for a 6g batch and not too low for when I make a 12g batch. I did have to put another ball valve on there. My whirlpool tube is adjustable as I used two silicone o-rings for the compression fitting instead of a stainless ferrule.
I put mine at the top with a cam lock QD and no valve. On the inside I have a 90 with a piece of pipe bent to keep the flow to the side of the keg. I don't have it soldered where the street elbow fits into the adapter at the coupler but it's a tight fit which allows me to adjust the flow up or down when I install it. Prob don't really need the adjustment and I prob keep it in the same spot but it works well for whatever batch size I'm doing.
Well, place it down where your 4 gallon mark or lower. It won't affect your 12 gallon boils. I dropped mine down really low because back when I built it there was a lot of talk of hot side aeration, and I also pimp my mash liquid through the same fitting, so I didn't want it to splash.