As hearing aid target derivations and associated amplification schemes become more sophisticated, the accuracy of the information used to derive the initial targets becomes more important. Threshold measures of hearing are routinely referenced to dB HL. Hearing aid targets are typically referenced to dB SPL. Procedures vary, but most schemes include some method for "predicting" TM SPL from dB HL measures. This presentation will focus on the effects of such predictions. Inaccuracies using "average ear" transformations are mediated by individual differences in ear canal acoustics, impedance at the TM, and transducer type and placement. Data that was obtained using a system that enables in-ear calibration using standard transducers (e.g. TDH, ER-3A) will be presented. Consistent with previous reports in the literature, the data reveal that individual subject variance can exceed 20dB at frequencies throughout the audiometric range using any standard transducer. The results suggest that application of any average transformation to predict TM SPL is only as accurate as the degree to which the ear under test has acoustic characteristics equal to those included in the derivation of the transformation. Direct measurement of the extent to which the canal acoustics of the ear under test differ from "average" provides a simple and accurate means by which TM SPL can be determined.