Jaybird,
You didn't mention what kind of filter you have. The DE trick only works with a sand filter. If you do have one then it most likely is not filtering the fine stuff which could be pollen, dust, dead algae. As for the DE, the trick is to add enough to raise the pressure about 1 psi. The amount you add depends on the size of the filter and other factors. Start with 1/3-1/2 cup, mix in a pail of water, add to the skimmer, wait 20 minutes and see what the pressure rise is. Repeat as necessary. Once you're 'calibrated' you just add that much at once. You have to add any time you backwash because the DE is washed out.
Al