[en] This report is the third in a series which takes into account the boundary condition in electromagnetic problems such as used by the program POISSON. Here we extend the analysis to permit the use of an elliptical boundary both for two-dimensional and axisymmetric cylindrical problems. The use of an elliptical boundary instead of a circular one can reduce the mesh size when using the program POISSON and thereby save computing time. Saving cpu time can be significant for problems such as the 2-in-1 dipole proposed for the SSC or other magnets such as solenoids. We therefore expect the use of an elliptical boundary to be more general and the previous spherical boundary solution to be a special case