[en] Spin motion of an electron depends sensitively on the details of its trajectory as it travels through all the electromagnetic fields in the storage ring. Various simplifying models seem to agree with each other only for ''smooth'' machines. For strong focusing machines such as SPEAR and PEP, one needs a much more accurate model in order to estimate the expected level of polarization. In this paper, we present a formalism which overcomes some of the difficulties. It seems that most known linear depolarization effects may be evaluated using this method. A computer program using the theoretical methods presented here is being developed and the numerical results will be presented in a subsequent report. 15 refs