Why Front-to-side ratio is read off the polar plot by comparing the peak of the main lobe with the response at 90 degrees away from it. In Figure E9-1 the main lobe peak sits on the outer 0 dB ring, and at the 90 and 270 degree points the pattern has fallen to about the 14 dB ring, so the ratio is roughly 14 dB. The rings on these plots are calibrated in dB below the maximum, so you simply count down from the outer edge to where the trace crosses the side direction.