Same story as another old thread:
Adding DE to Sand Filter - 3 cups and no pressure rise?
I've got a 26" sand filter (Pentair 145333 SD-80) and a 1hp single speed pump.
Started at 14.5 psi after a backwash. Added a 1/4 cup of DE, another 1/4, then 1/2 cups until I got to 3 cups. Pressure...