Is this residential or commercial? It depends on the why you are calculating it but the ceramic porcelain , I believe, are capable of 600 W-- They would never see that and the most I have seen in then is 200 watts but that is very unusual.
I would use 3 watts / sq.ft in a residence unless there is some obvious problems with that. Art 2220.14(D) states to use the max VA rating of the fixture for which it is rated