Some theoretical determinations, and with greater generality, experimental research show that, in most cases of practical interest, the law of the flux for a compressible gas, being substantially assimilable to a perfect gas as a function of pressure drop between the extremes of a piping, including concentrated and distributed resistances, can be well approximated by an ellipse. The said ellipse can be determined in different ways, from the flow values for some values of pressure drop through the piping, theoretically or experimentally determined. Practical methods are suggested for determining the ellipse coordinates, which seem to assure simplicity of the advantages of simplicity without detriment to accuracy.