The PAR readings you get in an aquarium vary with where in the tank you measure it. It will be highest at the water surface and lowest at the substrate. I like to use the PAR reading at the substrate to characterize how much light you have - at least that is just one number, easy to measure. I haven't kept any discus, but I believe they much prefer lower light intensity, which goes well with sword plants, vals, crypts, anubias, etc. If you stick to low light intensity you can either use CO2 or not, but if you do use it, the plants will grow much faster and in better health, and, for low light, you don't need to jack up the CO2 concentration so high that it threatens the fish's ability to live with it. Based on that I would try for about 25-35 micromols per square meter per second (25-35 PAR) at the substrate over most of the substrate. If you can get this intensity with the light hanging a foot above the tank, that is even better. Then, if you find you want more light you can just lower the light closer to the top of the tank.