I did not take the top off of the resin tank. I shined a flashlight behind it and saw the level 7/8 or so full at one point. The lowest I saw it (guessing that it was actually the resin only) was a little over 1/2 full - again, just shining a flashlight from behind the resin tank to see the level. It never dropped below a little over 1/2 full.
I 'guessed' that when the level was higher, that water filled the area above the resin.
I 'guessed' that when the level was higher, that water filled the area above the resin.