The PAR data for the 48" Fuge Ray at 18" is 48. That's just enough light to be considered medium light, but likely more low light. Then factor in that data being recorded in open air, the diffusion of your glass top and the clarity of your water and you're likely looking at a lot less than 48 at your substrate.
In the D.C., Maryland, or NOVA area? Come check out The Greater Washington Aquatic Plants Association! www.gwapa.org