Anybody with the same (frustrating) experience?
In defense of the WPG metric it is simply easy to calculate. Sum the value for watts on all bulbs (Assume ballast power factor of 1 and efficiency of all fluorescents is the same*) and then the tank size listed on the box (ignoring that it is calculated based on outside dimensions, full to the top, and no substrate) and divide!
There are several candidates for replacement but none has really caught on.
After a while you learn to watch the plants and give them what they need.
*Assumptions as FAR from ideal!