The Magnesium II core‐to‐wing ratio (also known as the Mg II index) is a widely used proxy for ultraviolet solar spectral irradiance variability. We have developed a new algorithm for calculating this index from the SOLar‐STellar Irradiance Comparison Experiment (SOLSTICE) on the SOlar Radiation and Climate Experiment (SORCE). The new method uses weighted sums of the core and wing regions of the spectrum calculated from the daily level three high‐resolution spectrum. We also describe a new method of scaling the results in order to compare to other measurements. This new method scales each data set to a standard spectral resolution rather than scaling to other data sets during periods of overlapping measurements. Finally, we quantify the effect of long‐term instrument degradation on the Mg II index. In the case of SORCE SOLSTICE, using uncorrected data would produce an error of less than 0.6% of the solar cycle amplitude over a decade.