Low-cycle fatigue damage is examined using a specific theoretical Stage II crack growth model, adapted to series 300 austenitic steels. This method builds upon previous 3D Dislocation Dynamics (DD) studies, deriving an expression for crack growth rate ($da/dN$) that incorporates the cyclic plasticity mechanisms governing grain-scale Crack-Tip Opening Displacement (CTOD). The objective of this research is to assess the efficacy of this particular model in predicting Stage II crack growth across macroscopic poly-crystals. The transition from microscale (grain) to macroscale is facilitated through a distinct procedure, which involves systematic variation of the sub-grain model inputs using relevant polycrystalline EBSD data. The outcomes of these calculations are presented as fatigue crack growth rate plots and fatigue lifetime plots. The findings align with pertinent experimental observations within the LCF regime, both in terms of absolute values and variability. The methodology employed here seeks to provide robust quantitative data to bolster fatigue design activities, particularly within the context of fusion reactor technology.