For an example hight of the water tank column is 15m and tank hight is 3m
Then
Total Head is 15m / 18m
I think 18m
What you say about this?

What do you mean?
At a guess 18 sounds great..?

Your question is vague, shaarim. Could you post the original question please?

Get a plumber to install a transparent standpipe beside the tank. Then you can see the water level -- the actual head at that time.