In this paper, an extended definition of the Strehl ratio is introduced for Bessel-Gauss (BGB) beams, which, in contrast to classical beams, are not defined at a single point but along the optical axis. For that purpose, the Fresnel diffraction integral is solved analytically to calculate the on-axis field of a BGB that is perturbed by spherical aberration, astigmatism, and coma. Certain approximations are introduced, discussed, and tested against a rigorous propagation method. The derived expression is applied to tolerancing of an example system.