calculate power radiated

Hello

I desine a horn antenna by hfss and now I want to calculate the NFD and plot it versus frequency. NFD means near field directiviy of antenna, it describe by this equation "NFD=Pf/Pt" which Pf is the power radiated from the front face of the antenna and Pt is the total power radiated through all the surface of the air box enclosed antenna

I know that in can be explaned in the calculator but i don't know how ?
I can't work by calculator easily and read the fullbook but didn't find enything helpfull

can enybody help me?
thanks a lot