External-field effects on persistent-hole spectra in disordered solids can be described with a stochastic theory of dye-matrix interaction. For obtaining analytical results, usually two approximations are performed in the calculation, the continuum approximation which is valid for sufficiently low matrix densities, and the Gaussian approximation which is valid for high densities. Hence, the validity range of the results is not clear a priori. The paper shows for the special case of pressure tuning that both approximations must be relaxed in a proper description of the physical situation. Especially the continuum approximation which consists in neglecting any correlations between the matrix molecules, leads to quantitatively incorrect results.