5x the tank volume through the refugium, at least 20x turnover in the refugium. For chaeto I go more flow.
My 25g refugium has 2 MJ 1200's and the drain from the display, couldn't be happier with it. It is the sole means of filtration for my 34g and I pull a tremendous amount of chaeto out a month.
Get some 6500K bulbs or so as well. You will be stoked on it.
Chaeto has the tendency to collect detritus and such IME. The higher flow frees this as well as making the chaeto grow better IME.
Calfo suggest 20x an hour in the refugium as well.