What is the maximum rate of change in your tank? The return and out put on a tank my vary. In other words, you may not always be pumping out and taking in the same amounts. So, subtract the output from the return, this would be your maximum rate of change at any given time. This can then be used with the pressure flow curve to determine the size breather you will need. Also, what is the environment like very humid or dusty? This will also help determine the proper size breather. As these conditions can reduce the breather’s life quicker then dry and less dusty environments.