Your fine! First off, forget the watts per gallon. Just put it out of your head. It was only somewhat relevant for T8 and early PC bulbs. With T5 and LED's, it doesn't mean anything.
PAR is what you want to know. It is the measurement of how much actual light is hitting the tank. Most people go for a PAR level of 40-50 at the substrate for Med-High light tanks. But here's the catch, PAR meters are on the expensive side. So what I did is look at what LED's and setup's other people have used and just tweak a little from there for my own needs/tastes to make a LED light for myself.
I learned a lot on
The Planted Tank - Articles, Forums, Pictures, Links. They have the guys that know this hobby inside and out, and you can look though tons of tanks and lights and see what's working for other people. Don't give up though, I got really frustrated trying to learn about LED lighting before I finally had my 'AhHa!' moment and it all made sense. I don't have a PAR meter, so I just found some lights people were using for similar tanks, and just did my own thing with them.
Rapid LED Home Page is where I ordered all my stuff for the LED's. They have a good selection of what actually works without trying to sell you something that you don't actually need.
And yes, they will grow plants rather well....at least that's the rumor.
Off the top of my head, you'll want at least 12-15 LED's (Cree is a brand name, but is really popular. I thought it was a type of LED for the longest time), but probably more depending on the light levels you want. On larger tanks like this, people also like to mix up the lights, getting some white lights but also some blue's to run as well to add more to the color. And don't skimp on good drivers for the LEDs.
I'm still new at LED's, but more than happy to help with any questions you may have. I'll volunteer ChichlidMan as well for help, as he built a very nice array for a non-planted 100g discus tank.