The prediction of backgrounds to new physics signals in topologies with large missing transverse energy and jets is important to new physics searches at the LHC. Following a CMS study, we investigate theoretical issues in using measurements of $\gamma 2$-jet production to predict the irreducible background to searches for missing energy plus two jets that originates from $Z 2$-jet production where the $Z$ boson decays to neutrinos. We compute ratios of $\gamma 2$-jet to $Z 2$-jet production cross sections and kinematic distributions at next-to-leading order in $\alpha_s$, as well as using a parton shower matched to leading-order matrix elements. We find that the ratios obtained in the two approximations are quite similar, making $\gamma 2$-jet production a theoretically reliable estimator for the missing energy plus two jets background. We employ a Frixione-style photon isolation, but we also show that for isolated prompt photon production at high transverse momentum the difference between this criterion and the standard cone isolation used by CMS is small. Show Partial Abstract