Hi Mike,
Not sure if these were the papers you were referencing last week, but the calculations look to be in the following two references :
Factors Affecting Contrast and Resolution in the Scanning Electron Microscope†
T. E. Evekhart , O. O. Wells & C. W. Oatley
Pages 97-111 | Received 21 Jul 1959, Published online: 26 Apr 2007
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https://doi.org/10.1080/00207215908937191And
Idealized spatial emission distribution of secondary electrons
Journal of Applied Physics 43, 3707 (1972);
https://doi.org/10.1063/1.1661794T. E. Everhart and M. S. Chung
Not sure what's going on with the Everhart/Evekhart - assume this is a typo from the document scanning.