The present report demonstrates that for at least four pharmaceutical powders, the variation in fill weight associated with a vacuum/purge filling port is correlated with the length-diameter ratio of that port. This relationship has been mathematically modeled, and a design curve based on production data is presented, which depicts this relationship over a wide range of length-diameter ratios. For powders with properties similar to those presented, the design curve may be used to determine the dimensions of the port which will yield acceptable process weight control. Fill weight variances also can be predicted given a fixed port diameter. For other powders, the model can be used to create design curves with a few data points.