I'd drain some water at least a foot and then test for CYA after filing back up. Keep adding some chlorine to the pool so the algae does not go crazy. SLAM with a CYA of 80 is not cheap with 26,000 gallons. Try to get your CYA to 40.
OrThe CYA test is not easy-- but you can use the same solution and do the test a few times. Or have your spouse do it and compare your results.
2 hrs hand vacuuming out all the green stuff tonight - adding 6 gallons of liquid chlorine. Leaving the pump running all night so will see how it is in the morning. CYA on regular test still shows 90, but on diluted test is closer to 60 - so maybe somewhere in between.
As I've been vacuuming, I have also backwashed a few times - quick question there - if I'm just backwashing to rid the filter of all the green algae, I don't need to add DE each time do I? I did add it when I was finished.