The Grunn equation:.times..times..rho. ##EQU00001## is modified to accurately predict depth of electron beam penetration into a target material. A two-layer stack is formed comprising a thickness of the target material overlying a detection material exhibiting greater sensitivity to the electron beam than the target material. The target material is exposed to electron beam radiation of different energies, with the threshold energy resulting in a changed physical property of the detection material below a predetermined value marking a penetration depth corresponding to the target material thickness. Utilizing the threshold energy (Vacc), the target material thickness (Depth), and the known target material density (.rho.), the numerical power "n" of the Grunn equation is calculated to fit experimental results. So modified, the Grunn equation accurately predicts the depth of penetration of electron beams of varying energies into the target material.