I exchanged PM several times with wa8vzq, and he told me about VOR theory.
Now, I write the my conclusion of this topic.
Formula that I presented before might be correct.
and (PI^2)/8 might be calibration factor of SB RF Power.
because VOR SB is not simple sine wave(sometimes called CW).It's like Double balanced modulation of 720Hz.(This is for japan)
(and Ideal shape is "(sin x)/x". but this is not Continuousness, so it's very difficult to make this wave by real Transmitter. but by using ROM can...omission
So,This calibration factor is used to calculate the RMS(or avg) of SB power.
Though it was not settled well, I think that I end by this.
Thanks for being interested!