A realistic mass transfer number that takes into account complex modes of heat transfer is determined from the local standoff distance of a boundary layer flame. The standoff distance at each stage of the propagation and as afunction of the streamwise coordinate is obtained by means of video recordings. A previously developed theoretical model is used to calculate the standoff distance as a function of the mass transfer number ("B-number"). Comparison of the predictions with the experimental values allows extraction of the evolution of a realistic mass transfer number as a function of the streamwise coordinate. Such an evaluation can be used as an effective material flammability criterion as well as a means to determine the relative importance of different heat transfer modes during upward flame spread. Validation of this methodology is done by means of microgravity experiments that use PMMA as fuel, and a critical value of this realistic mass transfer number is consequently established as a criterion for low strain quenching of microgravity diffusion flames.