Correlations of data characterizing arrays of subcritical units that interact to render whole arrays critical show that the resulting thickness of fissile material solution contained in an array, when “smeared” over the base area of that array, can be compared to the thickness of a single homogeneous unit having a composition and shape similar to that of the array. The correlations resulted in curves that can be used to predict critical arrays of solutions or to determine safe spacing using appropriate safety factors. Although the same relationship between array-smeared thickness and single-unit thickness does not appear to apply to metal systems, a useful curve that guides the determination of criteria for adequate spacing is developed.