It depends on who you talk to. Some HOB manufacurers think that less than 1% of the tank volume is enough space (hense the 1" rule). On the fish boards, the monkey-see-monkey-do answer is 30%. Asian fish keepers frequently use sumps that are 50% or larger.
Since we are talking about a 400g tank, thirty percent would be 134 gallons. That is the upper end unless you are leaning toward Asian type stocking. At the very least, shoot for between 10-20%. If you are going to house messy eaters, heavy waste producers, or are going to heavily stock the tank, then go as close to thirty percent as you can.
I am using a sump that is 10% of the volume of my tank with great success. I am also efficiently using the space and using pot scrubbers because of their high surface area to volume ratio.
