It's the Weight in ounces divided by the Area in sq feet. The area is the average Chord times the Span divided by 144 sq in/sq ft. So....

W/(Chord x Span/144) = WL

With the answer in this case as ounces per sq foot.

The wing loading for flying is largely a subjective matter. The plane will fly with a VERY wide range of wing loadings but for something like a Laser light is right. Adding much more will adversley affect the aerobatic performace. How much you add before it gets beyond YOUR tolerance and goes from being a fun model to fly to one that's just an accident looking for a place to occur is up to you. I'm sure you could stick an extra 5 lbs of gear in it and it would still be safe to fly but it would not be FUN anymore because it would need to fly so much faster for takeoff and landing to prevent stalling.