Fig 1 presents a plot of the new quantity vs for a variety of photon spectra.
Several things are clear.
The second observation is that, as expected, for the foil 30 cm from the phantom surface, the contamination is significantly higher. For values of less than the respective thresholds, . Since the measurement with the lead foil is no more difficult than the measurement without the lead foil in place, it is probably worth using the lead foil for 10 MV machines.