We prove a concise factor-of-two estimate for the failure-rate of optimally distinguishing an arbitrary ensemble of mixed quantum states, generalizing work of Holevo [Theor. Probab. Appl. 23, 411 (1978)] and Curlander [Ph.D. Thesis, MIT, 1979]. A modification of the minimal principle of Concha and Poor [Proceedings of the 6th International Conference on Quantum Communication, Measurement, and Computing (Rinton, Princeton, NJ, 2003)] is used to derive a sub-optimal measurement which has an error rate within a factor of two of the optimal by construction. This measurement is quadratically weighted, and has appeared as the first iterate of a sequence of measurements proposed by Ježek,Řeháček, and Fiurášek [Phys. Rev. A 65, 060301]. Unlike the so-called "pretty good" measurement, it coincides with Holevo's asymptotically-optimal measurement in the case of non-equiprobable pure states. A quadraticallyweighted version of the measurement bound by Barnum and Knill [J. Math. Phys. 43, 2097 (2002)] is proven. Bounds on the distinguishability of syndromes in the sense of Schumacher and Westmoreland [Phys. Rev. A 56, 131 (1997)] appear as a corollary. An appendix relates our bounds to the trace-Jensen inequality. * jonetyson@X.Y.Z, where X=post, Y=Harvard, Z=edu