The method for calculating the spin density of trapped normal muonium in diamond proposed by Hall and Mizukami (Chem. Phys. Letters 150 (1988) 23) is extended to silicon and germanium using crystalline electrostatic potentials obtained from X-ray diffraction data. The results suggest that the model is more appropriate for application to muonium in Si and Ge than in diamond. Likely shortcomings of the model diamond are discussed, and simple improvements suggested.