Vishal
There is no such fixed value that can be taken, Because with change in frequency the pattern on most antenna changes so does the max gain with direction with changing frequnecy. so my advice would be do it two ways
1. gain in the intended direction, for instance you want to your beam to face a certain direction with your max gain at theta=0, phi=0 direction(example only) then plot your gain values for these values
2. plot the radiation patterns first and check deviance from the max gain with changing frequency and direction and plot those values.
Hope that helps
Regards
Elchiquito