That calculator doesn't give a good result if you space the rows a lot further apart than the LED spacing. If you want to use evenly distributed LEDs over the whole top of the tank, I would use 3 rows of 5 LEDs, spaced 4 inches apart, with the rows 4 inches apart. And, I would use XR-E LEDs, http://www.dealextreme.com/p/cree-xr...ium-star-15943
with 40 degree optics. At about 600 mAmps you should get 60 micromols of PAR. It would probably take 2 LED Drivers, but maybe not. That should give you very uniform light, and the XR-E is fairly cheap now. DealExtreme is a pretty good store to deal with too.