+1 BL1. An easy to test how much water will flow out is to get a 5g bucket and let the water in your DT empty into the bucket until it stops flowing. Then either fill up 1g bottles and see how much or estimate how much water flows in.
Fill up your empty 20g tank to 13.5" high of water(the height of your baffles). Add that bucket of water into the tank and see if it overflows. If you still have some room left then you're in good shape. If it's really close to the top or overflows then you'll need to cut down the height of your baffles. Keep in mind that the tank will be empty so you'll to tank into consideration the water displacement for sand, lr, chaeto and pumps. I like to make sure I have at least 1" of room left when it's all said on done. That way it can handle a little more water if you should overfill with a top off or water change.
Also, depending on your overflow you can adjust the depth it sits in the water which will control how much it can let totally flow out of the DT.