I AM surprised...if you backwash your DE filter until the sight glass is clear, then you know you probably have no DE in it. So you add DE, leaving some "room" in case there is some left, until the pressure is where it's supposed to be...and you're done.

And I'm a sand filter guy.

I increase the sand filtering effectiveness several ways:
1) I have a filter clearly rated for a higher capacity than my pump.
2) I have a 2 speed pump I run on "low" most of the time--this increases the difference between the pump and filter and filter is more effective.
3) I add a cup of DE powder after I backwash, enough to raise pressure 4-5 #, no more (see Poconos) which catches more fine stuff.
4) I use "skimmer socks" in my skimmer to catch a portion of the fine stuff there.
5) I run my Dolphin overnight once a week. The Dolphin has as fine, if not finer, a filtering as a DE. It's high-flow, and runs for 8 hours as it's vacuuming, "polishing" the water.
6) I wear out-of-focus glasses so everything is fuzzy and I can't see if the water isn't perfectly clear!