Re: LOHAN ideas..
In the spirit of KISS - and I know it's nowhere near as fun as GPS or pressure related ideas, why don't you just set a timer to trigger the launch after the time you estimate it'll take the the balloon to reach roughly the desired height? It won't be an exact science but it should be close enough for jazz. If you have any data from PARIS on how long it took the payload to reach it's max height, that should help you make an educated guess as to when to set the timer to go bang.
It sounds too easy, am I being thick?