Hi Nirvani,
You don't saw a hole in glass, you cut a hole.
Anyway, where do you stay. Every difficult for pple to direct you to the nearest location to get the hole cut.
Numbers of hours is just a basic guidline.
Like what you have say it all depend on
1. Capacity of tank.
2. Type of lighting.
3. Hours of lighting on.
4. Length of pipping between the chiller and the tank.
Updated
chiller.htm
Someone did posted sometime back the the packaging of the new batch and the old is different. Unless you have bought the chiller, you wouldn't be able to open the parts to check right?