It depends on whether you overhang or place the light on the tank top. If you over hang the light a small distance above the tank, you get more uniform aerial coverage with trade off of wasting light shining outside the footprint and a reduction of light intensity by the inverse square distance law. If you place the light directly on the tank top, you maximize the light intensity reaching the footprint with trade off of less uniform coverage, and the closer it is to the light source, the less uniform the distribution. It also depends on the type of light. LED is directional light and the non uniform factor is far greater than florescent.
I place LED strip lights on glass top with additional submersible LED tubes tied with fish line to the front, side and back rims inside the tank to supplement light distribution. A glimpse on the Submersible LED tubes can be seen on the side view of my 75g, which are available at all lengths and color from Ebay.
Last edited by Tiger15; 05-28-2020 at 01:53 PM.