  A 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 $\phi_{\rm ord} = -\nu$ near the ordinary transition.
