  We present a strategy for estimating the error of truncated functional flow
equations. While the basic functional renormalization group equation is exact,
approximated solutions by means of truncations do not only depend on the choice
of the retained information, but also on the precise definition of the
truncation. Therefore, results depend on specification parameters that can be
used to quantify the error of a given truncation. We demonstrate this for the
BCS-BEC crossover in ultracold atoms. Within a simple truncation the precise
definition of the frequency dependence of the truncated propagator affects the
results, indicating a shortcoming of the choice of a frequency independent
cutoff function.
