Hmm, haven't seen the website. My suggestion would be to size the first section as small as you can to still fit the skimmer, and size the last chamber as large as you can to allow for evaporation and top off. If you are planning on running an auto top off system that isn't as critical... but still you will want some volume to your return section. Whatever is left is the refugium.
In my 20, I think I have about 8" for the inlet/skimmer, 12" for the refugium, and 10" for the return system. It works out well, but I have an ATO. With the ATO off, I lose about 2" of water per day in the return section.