The usual method for designing bandstop filters is the reactance transformation from a lowpass prototype. When the filter specifications do not satisfy geometrical symmetry, the choice of stopband central frequency constitutes an interesting problem. This letter shows that the mean square of the stopband edges is the stopband central frequency that leads to the lowest selectivity parameter of the prototype.
Mariño, J.B., Nadeu, C., Masgrau, E. Design of bandstop filters by reactance transformation. "Electronics Letters", 31 Gener 1980, vol. 16, núm. 3, p. 105-106.