Tony73,
My guess of 5% of the water running through the refugium may be off a little. Maybe 10%??? I would think you would want the water running slow enough through so the macro algae have time to absorb nitrates and other nutrients from the water that enters and exits the chamber.
There may be a formula to determine the correct or best size. However, the refugium has many advantages other then housing macro algae as we are all aware of.
Personally, I am still on the fence as to whether macro algae makes a good addition to a SPS reef tank. I have had both in the past, and really could not tell a difference. My concern now is I carbon dose my tank, and I am not sure macro is needed and if so, what impact will it have on the water quality. Macro does secrete tannins and probably could introduce toxins in the water. Very much so in the event of a die off.
I had planned on filling this chamber with bioballs, as to further build up my bacterial count in the system. Bioballs offer only a larger surface area for bacterial growth when fully submerged, and do not throw off the balance and produce nitrates. Only ,when they are heavily oxygenated and the water running over them is broken down into droplets.
I decided on live rock instead and to get a copepod population going with some zooplankton for my corals. I may try some cheato or other macro at some point, but for now I am happy with the progression.