The only RELIABLE way I can find to trigger an Alert Interceptor is with a react range >=55nm, with a border crossing event.
Without a border crossing, I get random results with a react range of 70nm. I even slowed my flight profile down to 280knots, A9.9, to give the AI more time...but the results are random. Sometimes the flight takes off, sometimes it never appears.
I'm guessing that there is a random factor (disregarding the percentage in the CREATE PLANE MB UI) built into the Alert Intercept routine, which is only overridden by a fence-crossing event, if the react range is greater than the range to the border from the airfield.
Again, if anyone has a mission, or has flown a stock mission, that would contradict the above, PLEASE let me know about it.