Your tank depth is 16 inch so your most effective PAR(Photosynthetically Active Radiation), and by extension PUR(Photosynthetically Usable Radiation), will be found at 6400-6700k.
As for Lumens per gallon, you want to step back one and determine your Lumens per watt first as that will establish how effective your light is for the amount of energy consumed. From there you establish Lumens per gallon via watts per gallon and between that and PAR you will receive the best choice for aquatic plants.
If you want to get really fancy, post your hardness and we can get so specific that you'll have to grow your hair long, gain 150 pounds, and never leave your mom's basement.