The dE/dx vs P curve becomes almost universal if one uses dE/dX vs βγ(=P/M), where X=ρx, measured in [g]/[cm]2.
The minimum is for βγ=3-3.5 (slightly dependent on Z; higher Z gives earlier minimum). Height of the minimum (dE/dX units): 1-2 MeV/(g/cm2).
Thickness of the atmosphere: 1032 g/cm2.