I don't think you can pin down the amount of water a VFT will need in mL. The temperature and humidity will affect that. I keep about ½" (~1.2cm) of water in the tray. Occasionally I let the tray dry up, but watch my plants soil carefully when I do. You don't want to let the soil dry out or it could kill the flytrap. As long as there is enough water in the tray to keep the soil from drying out during the day, you should be fine.