Powder refinement in a high energy planetary mill is investigated experimentally and analyzed using discrete element modeling (DEM). Experimental parameter tracking the progress in the material refinement, specific milling dose, is introduced as a product of the charge ratio and milling time. This parameter is determined experimentally for reactive milling of boron and titanium powders for which the time of initiation of a self-sustained reaction is measured for different milling condition. It is assumed that the reaction is always initiated when the same degree of structural refinement is achieved. A parameter analogous to the specific milling dose is considered based on the DEM calculations. It is established that for the milling conditions used for reactive milling in this project, the primary type of ball motion is their rolling along the vial's circumferential surface. The reciprocal of the rate of energy dissipation resulting from the ball rolling is used to create the DEM analog of the specific milling dose which correlates well with experimental observations. The usefulness of the DEM model is established in predicting the milling conditions for varied process parameters for mechanical alloying or reactive milling performed in a planetary mill.