We have obtained the absolute absorption cross-sections of Yb-doped phosphate QX laser glass at 300 K from 800-1100 nm, and generated the emission cross-sections via the standard method of reciprocity. By generating nine detailed high-confidence fitting functions for the absorption cross-sections, we then applied reciprocity to the individual absorption functions and obtain, for the first time to our knowledge, nine corresponding detailed emission functions that when summed, display a more accurate representation of the emission cross-sections as a function of wavelength than the standard method. A deeper mathematical understanding of reciprocity is also presented, along with derived Yb:QX Stark levels.