You want to sink the light as deep as possible whereby you can still see shrimp. You always want to dip in the darkest light field you and your guests can see in. So this cannot be an exact science as far as "depth of the light sinking". The variants from the different tide strengths play a factor too and can pull a light horizontal into the current.

My adjustable line is about 17 ft long. I like to dip in about 10 ft of water. I try to deploy the light 5 ft from the top water if the clarity is good. If the water is dirty then I need my light closer to the surface.

Kill zones are healthy when the shrimp come into the zone right and head straight towards your net. If the shrimp start to turn left or right or go deep before you can net them, your zone is problematic. This is usually attributable to a high intensity light field. Shrimp have lenses in the back of their eyes that amplify light, they have a sensitivity to strong light in the water and do react.

If the shrimp come at you like missiles & you can see them, your lights are at the right depth. If the shrimp turn away the closer they come to your net, you have light intensity issues that need resolved.

I don't want you to get focused on exact deployment numbers. I want you to watch the behavior of the shrimp. They will let you know if your kill zone is healthy or not.