I highly doubt that any rainstorm will wash the screenings out from under your pool, mine was built that way, and it actually helps drain the water away better than any dirt you'll be putting there.
Also, the reason I stated 2 feet beyond the edge of teh pool is so that the screenings can be graded away from the pool once it's done for better runoff, also, you probably want screenings right up to the outside of the butresses anyways.