The watts per gallon "rule" seems to apply reasonably to standard shaped tanks that are 20 gallon or more in volume. So, that's why I used a standard 20 gallon tank depth as a comparison. 2.4 watts per gallon on such a standard tank would be high light intensity if the bulbs are PC bulbs with a reflector as good as the AH Supply reflector. That is roughly based on my experience. Starting from there: your tank is about 14 inches from bulb to substrate. A 13 inch deep, standard tank would have the bulbs about 3 inches closer to the substrate. So, using the linear drop off of light from tubular bulbs, you should have the equavalent of 11/14 times 2.4 watts per gallon, or 1.9 watts per gallon. I would consider that moderate light intensity, except that your T5 bulbs are brighter than AH Supply PC bulbs. Based on that I would guess you have the equivalent of around 2 watts per gallon or a bit more. Raising the lights to get that to the equivalent of 1.8 watts per gallon would mean raising them to 2/1.8 times 14 inches, or 15.5 inches, so you might want to try raising them 1.5 inches.

This is nothing but crude estimating, but at least it tells you that you don't want to raise them a foot. And, it gives a feeling for the logical amount to raise them.

Things like this always remind me of back when I was working for NASA. One time one of my fellow engineers, with more experience and education than me derived an equation for how much heating a certain shape object would experience in a very hot high mach number jet of air. But, he needed some data to verify his equation. I did a test in a device I was trying to develop into a usable piece of testing equipment for just that kind of testing. I managed to get about one second of data, with not much accuracy, and gave the numbers to him. He fitted his equation, on log log paper to that single data point and wrote a paper about his method. I kept trying to tell him he couldn't do that, but he just said, "but I did". That was when I discovered the magic of log log graphing data - you can prove anything that way. That is what this playing with light intensity is like.