Here's how to eyeball it:
To fill your bong with water . . .
Downstem slits should all be underwater. If you don't have any fancy percolators - you can stop here! Usually about an inch or so of water. You can fill these through the mouthpiece or the hole where the downstem sits.
Click here if you have no clue what a downstem is.
If you have 1 perc: that perc should be underwater with all it's holes covered.
Click here to find out what perc bongs are all about.
- If you have 2 percs: cover both percs with water so that all the slits and holes are submerged. Fill from the mouthpiece, not the joint. Water should trickle down from the top perc (if not - blow into it or tilt the bong a bit until it covers the lower perc).
The Balancing Act: The trick is to submerge all the slits that air will flow through so it gets filtered through the water before you inhale. But if you're tasting bongwater as it splashes your lips -- you're filling the water too high or pulling too hard. Every cool bong is an art, take the time to get to know yours!