The equivalence of range and frequency averaging of acoustic propagation model results, based on the similarity of their analytic forms in mode calculations, was shown by Harrison [J. Acoust. Soc. Am. 97, 1314-1317 (1995)]. Here it is shown how oceanographic measurement errors and receiver bandwidth can be mapped into uncertainty in the number of modes being propagated. This can be mapped into range boundaries for averaging calculations, thereby giving upper and lower confidence boundaries for frequency-averaged transmission loss calculations. Examples of the application of this technique to synthetic data, where the measurement uncertainties are known and deliberately included, are shown.