But I see 2 drains....one on the right and one on the left?
It's hard to say how much evap you will see, and it will vary through the seasons, with turning the heat and a/c on and temp/humidity changes. you are just going to have to try it and hope for the best.
As for a 12 x 12 refugium, I suppose any refugium will be a help. You have to use what space you have.