  The purpose of this note is to point out a simple consequence of some earlier
work of the authors, "Hard Sard: Quantitative implicit function and extension
theorems for Lipschitz maps". For $f$, a Lipschitz function from a Euclidean
space into a metric space, we give quantitative estimates for how often the
pullback of the metric under $f$ is approximately a seminorm. This is a
quantitative version of Kirchheim's metric differentiation result from 1994.
Our result is in the form of a Carleson-type estimate.
