Relative error in the emergent Stokes I (first, third, and fifth rows) and Stokes Q (second, fourth, and sixth rows) as a function of the number of spatial grid points N_z, for the DELO-linear (left), DELOPAR (center), and DELO-parabolic (right) formal solvers combined with the trapezoidal, parabolic, spline and cubic Hermite optical depth conversion schemes in the absence of magnetic fields. The corresponding convergence rates k_X are reported in Table 2. The considered settings are: continuum (first and second rows), Sr I 4607 (third and fourth rows), and Ca I 4227 (fifth and sixth rows). The reference profiles are obtained with the cubic Hermite conversion scheme and DELO-parabolic, using N_z = 2561. The stars represent the relative error Eq. (17) obtained using the original domain extracted from the FAL-C atmospheric model.