a 6" wide screen should have about 200-250 GPH over it. If you pump is in the neighborhood of 800 GPH, and then add in loss (which might be dependent also on return pipe size - the larger the less loss, the smaller the greater loss) you could be more like 400 GPH. When you say "main pipe of a twin overflow" are you talking about a primary + emergency type of setup? So this is on the primary which gets most or all of the flow?