Chris,
The calculation of the drift values in Process Screening is described in more detail in this paper by John Sall: Scaling-up process characterization (starting at the bottom of the page numbered 73). A further reference (also given in the aforementioned paper) is this book by Hyndman, et al:
Hyndman, Koehler, Ord, and Snyder. 2008. Forecasting with exponential smoothing, the state space approach, Berlin: Springer Verlag.