spectrographic throughput

Optimistically, we (Blanton, Bolton, Burles, Masjedi, myself) decided that the PRIMUS prism spectroscopy of redshift unity galaxies will start to measure good redshifts as soon as we understand the throughput well. The throughput is not trivial to measure, and it depends on airmass, photometricity, and other factors. But we spent all day working on it and are very close to having some reasonable estimates. What I don't understand is what kind of data we need to take each PRIMUS night to ensure that, after the fact, we will be able to deteremine the throughput accurately enough for good redshift determination.