There are two things roughly covered by this watts per gallon measure, and those are Area to be lit and distance from light. Area to be lit is fairly simple, more area to be lit would need more light, thus a 20 long needs more light than a 10 gallon to have equal lighting.
One consideration for fluorescent lighting is available tube lengths. The 30" length tanks (like your 37) are awkward because they are inbetween two lengths of tube, one to short and the other to long for exact coverage of the length.
So that leaves distance from fixture to bottom of the tank. Light intensity drops off exponentially with distance (Intensity is roughly= 1/distance squared). So to have enough light at the substrate for carpeting plants in your tank would by necesity require high light because of the depth of the tank.
To give an example I had the amout of light you currently have on a regular 20 gallon. My tank is 6 inches less in length and 4 inches less water depth to light. I considered it very low light and experience proved I could only grow easy plants. No carpeting plants would grow in my tank. I also had that same amount of light on a 10 gallon tank and could begin to grow some bottom plants. To light your tank like my 10 gallon and be able to grow easy carpeting plants you would to at least triple the light you have and then it would be a struggle to get them to grow.
I hope this helps you understand your options and the previous posters suggestions, which I agree with.