Wide variations in the dose enhancement factor observed when milling silicon using Focused Ion Beam (FIB) XeF2 Gas Assisted Etching (GAE) prompted the development of a simple model of the GAE process. The model accounts for three material removal mechanisms: regular sputtering; gas-assisted sputtering; and spontaneous chemical reactions. An expression linking the dose enhancement factor, εd, to the gas and milling parameters has been derived. Experiments show that εd behaves as predicted; good quantitative agreement is achieved over wide ranges of milling parameters for εd values between 20X and 2500X. Conditions required to minimize variations in d and maximize material removal rates, M, are derived. It is shown that if the dose per unit area per raster is below a threshold value then εd and M depend only on the average current density J (the area of the box divided by the beam current). A consideration of the J regimes used for front-side and back-side FIB work shows why changes in εd have not previously been a problem but are inevitable when milling the large trenches characteristics of Flip Chip circuit modification work. While εd changes dramatically there is a region of J values for which M is approximately constant.