You will want to fill it up above the fritted disc perc.
How much you fill it above the fritted disc is your choice. Water level is definitely a personal preference. Some people like more water for more of a cooling effect, while others like less water.
Some people would fill it just right above the fritted disc, while others may fill it halfway between the fritted disc and the cyclone/vortex disc for more diffusion.
I think if you fill it more than half way between the fritted disc and the cyclone disc, the bubbles will stack too high and that will cause water to go up the mouthpiece tube and into your mouth. Yuck!
Wow this was my 500th message! SWEEEEETTTTTT
Enjoy the new piece