If you haven't seen much drop overnight for the past couple of days then I think you can let the chlorine drift down and keep it between 8-15 ALL the time.

You might want to run another CYA test to see if you can pinpoint more closely what your level is. But, this time mix 1/4 cup pool water with 1/4 cup distilled water. Then, run the CYA test with that and multiply your result by 2. If you still get a reading of 100, dilute it further --- one part pool water to two parts distilled and multiply results by 3,

Did you order the K1515? You need to. You will have no way to measure the higher chlorine levels accurately. You can't use an OTO kit exclusively with a high CYA pool. It just isn't accurate enough. (Reliable yes, but accurate no.) While you are ordering, go ahead and order some more CYA reagent R-0013. You'll need more of that as well. It, too, is available on the test kit link page:> http://pool9.net/tk/

If you don't have a lot of stuff being filtered out since the water is clear, when you backwash you aren't really removing much and thus you wouldn't expect the pressure to drop much afterwards. I think you are ok.