When to add more DE to sand filter

I was thinking of adding some DE to my sand filter. I've read posts on this forum regarding this and think I have it figured out when to add and how much to add. Basically 1/3-1 cup depending on when the filter's psi goes up 1lb.

My question is when to add more DE to the filter...is that just after I backwash?

I have some really fine "something" at the bottom of the pool. I had a sand problem last year with the filter putting sand into the pool. I fixed that problem and then it looked as if it was doing it again. So for 2 days I had some panty hose on the return and there wasn't a grain of sand in it. So either it's really fine dust that the filter can seem to filter or dirt/pollen from the lawn/trees.

I have used a sock on the skimmer basket but it was so nasty everyday to clean out so I took it off. Do you think the filter sock would make a difference?

Yes, add more DE after each backwash.