I also agree. I have a 46g bow and you're looking at a 21" depth on that. That's going to put your light right in between med & high and will depend on your reflector which one it leans toward. If when you look at the bulb reflector on your light you see that the reflector is like a mirror, it is fairly wide being significantly wiser than the bulb, and you see mutiple reflections of the bulb in it then you've got a high quality light.
I'm growing swords, anubias, dwarf hair grass, wisteria, moneywort, various crypts, and I used to have java moss. I have a good quality 4 bulb T5 but only run two at a time except for a 1/2 hour overlap and for a total of 7 hours. I also run pressurized CO2 system and a EI method of fert dosing. I was aiming at high growth.
Your suggested 8 hours will be a good place to start if you want to keep it a low to med maintenance tank. I agree with Bob though that you probably won't see the DHG filling in much without more light and CO2. A different cover plant might be an idea. I'd also discourage you from the java moss. It is a fairly low light plant and grows fairly easily but it grows all directions with no real shape to it. It can also be a algae magnet and when it gets loose it will plug your intakes. I ended up throwing it into a small low maintenance tank I have and the baby mollies seem to love it and it seems to like being there. But in there I don't really care what it looks like.