I have about 20" between the top of the gravel and the surface of the water. I'm still rocking the original T8 fluorescent hood with a supposedly plant friendly bulb but I know that it isn't enough.
Waterproof LED strips you say? I'm intrigued. That sounds awesome but shouldn't the light come from above the plants?
I have a great deal of experience with garden plants...but aquatic plants much less, and real lighting issues, almost nada! But the point is plants need light.
They can use light from where ever it comes from. I have been successful for two months at keeping low light plants very healthy in buckets by adding my aquarium light to the sides of the buckets as well as sitting it over the top of the buckets. They are the square cat litter plastic buckets (no cat litter). One is almost clear and the other is slightly opaque.
They are getting good light but from the side bottom where I lay the light hood on the floor facing into the bucket bottoms. I did this since they weren't getting light down there, and also they had bunches of duckweed and Salvinia Minima covering the top for sure blocking light altogether in one bucket! They are extra plants, I don't have room for in the tanks atm.
They are lower light plants, so it isn't exactly a miracle or anything just that they weren't getting enough light and now they are.
The leaves of aquarium plants and depth of the tank can make you loose the majority of available light once you reach the substrate.
If you have seen PAR charts for light fixtures and tanks you can easily see the dramatic loss from the top of the water then every, say 8-10 inches.
By the way there is a nice Finnex PAR chart thread here somewhere, I was looking at it last night.