Quality of the polynomial fit depends, among other things, on gamma-ray line identification. Typically, a few lines are identified over as large a channel range as possible and then used for an initial fit. Validity of the fit is verified by energy identification of other lines in the spectrum. It would be useful to list the initial set of lines (channel number and energy) used to define the polynomial. If a user has questions about the quality of the fit, this would be a good place to start.
Tom P. talked with Richard Starr who is ok with the plan I emailed you for specifying line identification in the data set documentation rather than in the dictionary.