The steering law model describes pointing device motion through constrained paths. Previous uses of the model are deficient because they are built using only error-free responses, ignoring altogether the path of the cursor. We correct this by proposing and validating a technique to include spatial variability, including errors. The technique is a variant of the well-known "effective target width" used in Fitts' law models. An experiment designed to test our technique demonstrates the improvement: Correlations are consistently higher when spatial variability is included in building the model. Suggestions to aid further development of the steering law model are included.