  To date no metal-free stars have been identified by direct observations. The
most common method of constraining their properties is searching the spectra of
the most metal-poor stars for the chemical elements created in the first stars
and their supernova. In this approach, modelled supernova yields are compared
to the observed abundance patterns in extremely metal-poor stars. The method
typically only uses the abundance ratios, i.e., the yields are diluted to the
observed level. Following the usual assumption of spherical symmetry we compute
a simple lower limit of the mass a supernova can mix with and find that it is
consistent with all published simulations of early chemical enrichment in the
interstellar medium. For three different cases, we demonstrate that this
dilution limit can change the conclusions from the abundance fitting. There is
a large discrepancy between the dilution found in simulations of SN explosions
in minihaloes and the dilution assumed in many abundance fits. Limiting the
dilution can significantly alter the likelihood of which supernovae are
possible progenitors of observed CEMP-no stars. In particular, some of the
faint, very low-yield SNe, which have been suggested as models for the
abundance pattern of SMSS0313-6708, cannot explain the measured metal
abundances, as their predicted metal yields are too small by two orders of
magnitude. Altogether, the new dilution model presented here emphasizes the
need to better understand the mixing and dilution behaviour of aspherical SNe.
