If you don't know what the pressure was when the filter and pool were clean, it's hard to know how much over that level you are. You don't have equipment details in your sig, but your other thread suggests a DE filter. I'd suggest just taking the filter apart, rinsing all the grids and cleaning out the basin, and reassembling and then adding the DE recommended by the manufacturer. There will be a window of time between when you add the DE to the skimmer, it's all distributed to the grids, when you can get a reference PSI. then you'll know when you get 25% over that. Backwashing DE is not an exact science; you add "60%" of the new install amount of DE. I gave up backwashing and am just doing yearly full clean out, but for a SLAM, you'll probably just backwash, then add DE to get the PSI back to the correct baseline. Which, again, is supposed to be about 60% of the original amount of DE