I've read that 1k to 2k gph per cubic ft of media works well.
Personally I'm not convinced you can have too little or too much, as long as your
shower can return the water effectively back to the pond. I've never seen a post
saying they're flowing too much water through their shower and it's not working.
Here's a old thread that might be of interest:
https://www.koiphen.com/forums/showt...amp-Flow-rates