The complexity of theoretical work in physics applications often forces severe neglections and crude approximations. Insufficient experimental knowledge of important parameters increases the uncertainty. In this situation the E-diagram method still allows to arrive at useful results through controlled choice of the approximations and sensible limitation of the aims. Choosing “unidirected” deviations only, existence limits can be calculated which restrict the existence of the system to a narrow region - the existence area - of the coordinate plane of variables under consideration. The example of the vacuum arc cathode demonstrates the procedure and its suitablility.