Here's another general rule of thumb that goes by light instead of watts....

1,500 to 2,500 Lumens/sq. ft for low light corals
4,000 to 5,000 for medium light corals
7,00 to 8,000 Lumens/ sq. ft. for high light corals

A general guide to figure out lumens...
50 Lumen/watt - NO Fluorescent
56 Lumen/watt – VHO Fluorescent
85 Lumen/watt – CFL Fluorescent
75 Lumen/watt – Metal Halide
85 Lumen/watt – HQI Metal Halide