The DE grids for a bottom manifold filter are all the same as there is no standpipe inside the tank to obstruct the grids. A DE filter with a bottom manifold is usually a Pentair SM/SMBW series filter that does not have a backwash or multi-port valve on the side of the filter tank. The square footage of your DE filter grids is easy to determine by looking at the height of your grids, just measure the height and double it. If you have a 24" tall DE grid, then your filter is 48 sq. ft. - can it get more simple than that?!