Aside from stability, the broad base of beaker bongs allows for more significant water capacity. Bongs have optimal water level because you need to get the flavor and filtration just right for a good hit. Generally speaking, there are two “landmarks” for checking the water level.
What you want to avoid when filing up your beaker bong is that nasty backsplash. Test your water level by filling up your beaker bong. Chug it without lighting up the herb in the bowl. If you had to chug hard and water reaches up to your lips, reduce the amount of water. Gradually reduce the water level until you get the right pull.
Measure the water level by placing the beaker bong on a flat surface. Get your ruler and note the water line from the base, as well as from the bottom of the perc. You can whip out your ruler every time you measure the water level until you become comfortable without it.
The best way to know the optimal water level for your beaker bong is to use it often. The more you use it, the more you will understand how it works. You’ll eventually find the optimal water level for your beaker bong depending on its size.