Rapid Transmitance calculations are required for the direct use of radiances in a forecast model. Rapid transmittance models work by fittng the transmittances calculated by an exacting but slow, line-by-line model. The accuracy of the fit depends on a number of factors including the profiles used, the gases considered, and the form of the data used for the fitting. This paper presents the results of several sutdies in these areas. One separates the effects of the profiles from the effects of the specific line-by-line model for the set of 32 atmospheres commonly used with LBLRTM model and the set of 48 profiles commonly used with the kCARTA model. One evaluates an alternative form of the model that allows more gas combinations to be used than has been the case with most of the models currently used. Results from these and related studies will be presented.