Fill until the chamber is two-thirds full or until two inches of the down stem are fully submerged.
Make sure the down stem is inserted, place your lips inside the mouthpiece, and take an even inhale.  If the water bubbles over the percolators and touches your lips, it’s a sign you have too much water. Remove the down stem and blow into the bong to push water out of the second perc. Then, tip the bong and pour a few inches out through the opening.  If you notice the bubbling sound is too quiet, fill the mouthpiece with a bit more water and try again.
You might have to repeat the pull test a few times before getting the water levels right. But after a few times filling your percolator bong, you’ll soon become an expert.