This paper estimates various interpolation/extrapolation functions for the theta parameters within the theta projection technique and assesses their ability to accurately predict the creep life of 1CrMoV rotor steel. The theta interpolation/extrapolation functions were estimated using short-term data collected at the Interdisciplinary Research Laboratories in Swansea and the accuracy of the predictions assessed using longer-term rupture times published by the National Research Institute of Metals in Japan. It was found that the theta interpolation/extrapolation function traditionally used by theta practioners was not the best predictor of long term life and that the most accurate long term life predictions were obtained using simpler functional forms. Further, it was found that more accurate lifetime predictions are obtained by estimating such interpolation/extrapolation functions using weighted rather than ordinary least squares.