Almost all applications of SAT solvers generate Boolean formulae from higher level expression graphs by encoding the semantics of each operation or relation into propositional logic. All non-trivial relations have many different possible encodings and the encoding used can have a major effect on the performance of the system. This paper gives an abstract satisfaction based formalisation of one aspect of encoding quality, the propagation strength, and shows that propagation complete SAT encodings can be modelled by our formalism and automatically computed for key operations. This allows a more rigorous approach to designing encodings as well as improved performance.