My DE filter runs around 12PSI when freshly back washed. I back wash when it gets to 20PSI and add 4 coffee cans of DE. Been using this system for years and pool is crystal clear. Don't get too hung up on the exact psi increase etc... I'm not sure of the difference between pool grade and food grade DE other than you can use food grade in your garden to control crawly thangs

Throw a couple cans in and forget about it until you see a pressure increase and then start from scratch .