We perform a numerical finite-size study for the static homogeneous diagonal conductivity σxx at the critical filling factor 3/2 for different microscopic realizations of the random impurity potential. The variation of σxx with the system size defines a scaling function. It turns out to be independent of the particular realization of disorder and also of the Landau-level index. However, the diagonal conductivity in the second-lowest Landau level varies strongly with disorder. The universal critical conductivity is recovered only asymptotically when the correlation length of the potential is increased.