Completely agree with ianho that your 1.2 watts/USgallon is plenty of light and with his good Kelvin comments (you are in luck when one of the plant guys like ianho answers!)
Have no idea how much you are in to plants or your experiences but I'll comment that once you've got the right amount up there shining down, the next consideration becomes the number of hours you leave it on. This will likely range between 4 hours and 12 hours. The amount that is optimal for you can take quite some time and experience to ultimately work out. Tanks can have wildly different water conditions (extremely low levels of ammonia, not quite so low ammonia levels etc.) and plant species, so it is usually quite different for everyone.
On the low side, 4 hours is often considered a pretty hard lower limit because vascular plants can not get their "machinery" up and running to make the necessary sugars in less than 4 hours and thus they would starve with less than that. The algae unfortunately have no such lower limit! At the other end, 10 to 12 hours, you are much more likely to promote an algae problem, if one is going to start up at all (that is not a given, just a possibility.)
So the trade-off becomes: Am I giving the plants too little light, leaving them a bit weak and slow. Or am I pumping too much light in there and having an algae problem. Most people will tell you to start at some number of hours, but the number they give you is based on experiences with their own tanks and that may be way off from your particular tank. In the end though, you do just have to start with 6 or 8 or 10 hours a day and just see what happens with the algae and then make perhaps slow half hour changes and then watch for some weeks. That is how you can arrive at an optimal lighting period for your tank.
I'm sure there are other things to say that I'm forgetting to mention.
~~waterdrop~~
/www.theplantedtank.co.uk/lighting.htm
