  Under the assumption that the photospheric quiet-Sun magnetic field is
turbulent, the cancellation function has previously been used to estimate the
true, resolution-independent mean, unsigned vertical flux
$<|B_z|>_{\mathrm{true}}$. We show that the presence of network elements,
noise, and seeing complicate the measurement of accurate cancellation functions
and their power-law exponents $\kappa$. Failure to exclude network elements
previously led to too low estimates of both the cancellation exponent $\kappa$
and of $<|B_z|>_{\mathrm{true}}$. However, both $\kappa$ and
$<|B_z|>_{\mathrm{true}}$ are over-estimated due to noise in magnetograms.
While no conclusive value can be derived with data from current instruments,
our {\it Hinode}/SP results of $\kappa\lessapprox0.38$ and
$<|B_z|>_{\mathrm{true}}\lessapprox 270 $gauss can be taken as upper bounds.
