What specific light fixture or fixtures are you using. Since my original charts of PAR vs distance were posted here I have seen a lot more data from a lot of different lights, and even more manufacturers are selling T5HO lights. So, I have posted a different set of charts at Lighting an Aquarium with PAR instead of Watts
The PAR you get depends on both the reflectors and the ballasts, and different manufacturers use different quality parts of both kinds.
I haven't tried hairgrass yet, at least not long enough to learn much about it. So, I can only go by what is posted here in guessing what it requires.
Since that tank is 18 inches front to back depth a single light fixture with closely spaced bulbs isn't likely to give uniform lighting over the whole substrate. But, if the light is high enough above the tank, it will give relatively uniform lighting. So, I suggest working with only two bulbs, closely spaced, if you have the light high above the tank, but using 2 widely spaced bulbs if the light is right at the top of the tank. But, that changes if you have one of the inefficient T5HO lights.