If I understand correctly, you added the granular stabilizer via the sock method. That means that it was dissolved before it went to your filter and little would be removed thru backwashing. And the liquid stabilizer is already mixed in the water
Since your normal/clean filter pressure is 10, then you should backwash around 12 or 13 psi.
Since your normal/clean filter pressure is 10, then you should backwash around 12 or 13 psi.