2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/29564A modified renormalization group equation for the inverse extrapolation length $c$ is derived by considering the phase shifts of order parameter fluctuations. The resulting non-linear equation is also derived using standard methods and some additional assumptions. The associated renormalized flow $c(l)$ exhibits the correct behavior near both the special and ordinary fixed points and in particular yields a canonical scaling of $c$ with cross-over exponent $ϕ_{\rm ord} = -ν$ near the ordinary transition.5 pages, REVTEX, submitted to Phys. Rev. LettStatistical Mechanicstext