Robin.
Pending a response from those with more expertise, I offer these thoughts.
A 50 watt plant light is near useless. (It has to do with "lumens", the measure of brightness. Next time you are in a store which sells 48" flourescents, look at the label on the box of the "noonday sun" models and compare this to the label for your plant light.)
With a plant light of any kind, the closer the better is the rule,
as long as it does not get the leaves too hot.
Most folks using lights for tropical trees indoors settle on about 16-18 hours a day.
Some species can do well with a lot less light.
What species is your tree?
