The depth really depends on the lake and where the fish are hanging. On JPP with the thermocline the fish are at or above it. Which right now is around 15'. I put my 2 foot lights just under the surface,and last night the bait ball was down to around 8 feet deep. We were fishing around 10 feet deep. Just under the bait. I always watch my sonar on the trolling motor and see what the fish and bait is doing and adjust accordingly.
I started out with the cheap bass pro lights about 5 years ago. But they just aren't as bright or durable. I think I paid $149 each for these LED lights but well worth it. Those things are BRIGHT!
But years ago I fished with my dad and we used car headlights floating on a cut out piece of styrofoam and we wore them out so in the end it doesn't really matter.