Generally the bigger the better. If you can somehow fit a 180G sump under a 10G tank, you should certainly do that.
I never heard of a formula but I've used a 40G sump with my 135G tank and now use 75G fuge with my 180G tank. If I had a house where my sump room would be below (in the basement) and my show tank on the first floor I would hook up multiple refugia possibly exceeding total volume of the show tank.