Is there a formula or system to calculate dmax in tissue from the energy of a photon beam? The ones listed in my textbook are as follows:

4 mv = 1cm
6 mv = 1.5 cm
10 mv = 2.5 cm
15 mv = 3 cm
18-20 mv = 3.5 cm
25 mv = 5 cm

Its as simple as dmax = mv/4 for everything up to 15. but then increased energy seems to have diminishing returns. Keep in mind also that these are approximations.