Well, the recommended amount is the amount of DE it takes to raise the clean pressure of your filter one pound. The small scoop I use adds about that or maybe alittle less. I mix the DE with water in a small pail and pour it slowly into the skimmer. The scoop is not the scoop they sell for putting DE in a DE filter. That kind of a scoop adds about a pound of DE material to the DE filter; which would be way to much for our size sand filters. (mine is 350#, 80# pea gravel and 280# sand) I know, 80 + 280 = 360. The sand came in 40# bags. I normally run the sand filter with alittle DE in it; but also have a DE filter in series that I run when the pool gets clear from the spring opening. If you put in to much DE and the pressure raised say 4#, it's not going to hurt anything. It still going to trap the dirt; and maybe work to good and you will have to backwash alot sooner. You're going to have to try different amounts to see what works best for your pool.
As for vacuuming and the flow-thru dirt; that's kind of hit or miss. You would add alittle DE to your sand filter before you vacuumed and when your filter hit the max pressure you want, you would then backwash and then add more DE to the filter. You are still going to get alittle flow-thru dirt.
Usually, to vacuum, I use my pressure side cleaner (Ray-Vac) to do the cleaning, have DE in the sand filter (with the DE filter running in series) and increase the pump to about 40gpm and shut off one skimmer. (just running on one skimmer and the bottom drain) Any fine dust-like-stuff that passes thru the Ray-Vac will usually get to the bottom drain and into the filters.
When I manually vacuum, I don't add DE to the filter. I'll vacuum using the sand filter only and when I am done, then I add some DE to the sand filter, cut in the DE filter and run only the bottom drain, increasing the pump to about 40gpm. This seems to get all the dust-like-stuff that went thru the filter after acouple of hours.
