HydroFarm 24" T5 lights and Footcandles

[Uechi]

I am in the process of setting up my Exo Terra Orchidarium. The tank is 24"x18"x24". I considered an HID, but they output too much heat. According to HydroFarm their 24" T5 lights with 6400K bulb put out about 2,280 Lumens. What I would like to know is what would the Foot Candles be for a setup of six 24" T5's assuming that the reflectors are close to the top of the tank and the distance from the plants to the multi light source is about 18". There is suppose to be a formula for figuring this out, but I don't have a clue. I am looking to have about 2,500 foot candles or better. Will this work or do I need to consider multiple compact fluorescents?