Interesting design for overflow anti-siphon! I'll have to do that with mine if I have time for it.
I always advise people to go with sump/refugium that is 30% or more of their tank size. The overflow will take in about 2-4% of your tank water and your sump should maintain at least 6" of water to keep pump completely submerged.
On my 125g, I have pre-sump and sump/refugium- both are 27g rubbermaid containers. Pre-sump allow for you to utilize skimmer, add additives, perform water change, test, etc. Sump/Refugium being last part, will allow for pods to return into main tank (chopped or intact, depends on pump type) You should have rocks and macroalgae in last part because it'll ensure your life in refugium have chance to pass through into main tank to feed your livestocks.
You can have your refugium connected to your tank seperately from sump. Just need other overflow box or use HOB refugium. I have CPR Aquafuge on my 33g breeder and works very well.