what is percentage impedence BW of this S11 vs freq curve foe wideband antenna

Usually impedance bandwidth is taken bellow -10dB limits of the return loss (VSWR better than 2:1)
So, in your case the bandwidth frequency is between m2 an m7 markers.

Hi, for percentage bandwidth you should calculate center frequency using equation fc=sqrt(fl*fh) and then percentage bandwidth will be (fh-fl)/fc. fl and fh are the points your response crosses threshold. usual threshold is equal to -10dB for return loss or equally 2:1 for VSWR. Somewhere threshold may be defined as -7.5dB return loss or even -6dB return loss or -14dB return loss

Wow, It looks nice. Four octave, 400%. It looks like open end of the waveguide, but they typically doesn't has so good matching.