One way to estimate how much reflected light your plants are actually getting: 4 ft fluorescent tubes radiate light cylindrically mostly. Thus one can determine a cylinder of a diameter (average distance to center of the plant) and about 5-6 ft long, assuming your plant is not too far away. Determine the surface area of this cylinder and the percentage of the area used by the plants at this distance. This equates to the same % as the wattage from original bulb. Subtract about 10-20% from the wattage of the bulb due to inefficiency of conversion from electricity to light. There will be some reflection from the back reflector on the light. Assume this to add approximately 50% to 100% more light on the plant area. Take into account the multiple fluorescent tubes you have in close proximity.

The result is an estimate of the number of watts actually hitting the plant which can then
be divided by the area of light the plant actually uses to get the watts/area.

Offered by Mike.