3 meter diameter panels @ 5 GHz gives a diffraction-limited beamwidth (FWHM) of lambda/D = (3e8/5e9)/3 * 180/pi = 1.5 degrees

0.75 degrees (1.5 degrees is the full angle) gives tan(0.75×pi/180)×(2×40) = 1 meter panels at receive (40 meter from transmit) to intercept the FWHM power. So a 3 meter receive panel will intercept more than the FWHM --> 70% seems like a realistic number for the percentage of power intercepted.