It's not clear to me why you want to add DE to your filter. It's useful if you suspect your sand filter has problems or needs more sand. But it's rather less useful if you are hoping to enhance filtration: if there's anything in the pool water that your sand filter is not stopping, you'll need to be ready to 'babysit' the filter once you add the DE. Pressure can rise very rapidly.
It *is* true that, in a sand filter, the dirt become part of the filter media, as the filter gradually collects smaller and smaller debris. This is where the 10# rise comes from. But there's a trade-off. As the pressure increases, flow rates drop. So, you end up filtering less and less water.
As far as when you backwash after adding DE, there is no 'magic' number, but it would be reasonable to backwash on the same pressure increase as you do normally.
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