well,
I don't mean to offend anyone but to be honest derasas are actually very forgiving in their light requirement. I know this b/c I've actually kept one for almost a year in a half under 260 watts of PC lights and it has grown significantly. It should be alright. The problem I had with mine was that no matter what I tried to do to keep it on the rock at midwater level, it practically (intentionally) made it way down to the substrate. So, I just left it there. Seems pretty happy.
I still own mine; it was doing very well under PC lights and is doing fine even now but I upgraded my system from 260 watts of PC to 216 watts of T-5's. <--not a down grade (just a side note).