We extend the absolute quantum efficiency (QE) calibration method, previously used only for photon counters, to analog detectors with a high dispersion of single-photon responses. Our approach is demonstrated on a reference-free measurement of a photomultiplier tube (PMT) cathode QE with 4% relative uncertainty. It involves not only measuring the biphoton field correlation function, but also a special approximation of the distribution of detector readings in order to determine the average number of photoelectrons and the average charge corresponding to a single photoelectron. Results of the calibration were verified in an independent experiment; the numbers of incident photons detected by the PMT and a single-photon counter are in good agreement.