This paper improves upon a standard method of determining the Flory-Huggins chi parameter, whereby experimental order-disorder transitions (ODTs) of symmetric diblock polymer melts are fit to the mean-field prediction, (chi N)(ODT) = 10.495. The improvement is achieved by switching to an accurate prediction of (chi N)(ODT) from Glaser et al. (Phys. Rev. Lett. 2014, 113, 068302), supplemented with corrections for the small degrees of polydispersity and compositional asymmetry that inevitably exist in real diblock polymers. The first correction is evaluated by simulating polydisperse diblocks over a wide range of invariant polymerization indices, and the second correction is extracted from analogous simulations for compositionally asymmetric diblocks by Ghasimakbari and Morse (Macromolecules 2020, 53, 7399). The resulting calibration method is then demonstrated on 19 different chemical pairs, using previously published experimental data. It provides a considerable increase in accuracy, but yet is nearly as simple to apply as the original version.