Is there any standard for ceiling height, because how to decide what should be the height false ceiling, I am in confusion , in my case total height is 8 meter and client is asking for false ceiling to be provided. But no specific height is given by client. What should false ceiling height to be taken in order to give optimum cooling load calculation.

8 meters (26 feet) is very high. A warehouse? Manufacturing?
You should discuss the needs, wishes, and regulations regarding height with the owner, and get it in writing. Or you could give calculations for about 4 or 5 heights.