interesting, im going to give that a go. How did you program it?
It's actually quite easy, once you've opened the X52 profile editor & selected the throttle, just follow the steps below:

You will notice that, although in my earlier post I said 20%, I have actually set it at 18% in the editor, this gives just a little bit more leeway when the throttle hits the indent at 20%, it helps to stop the reverse thrust coming on unintentionally.
IMPORTANT NOTE: When the throttle is set to activate F2, you must remember not to leave the throttle in that position when not using FS9, the PC thinks F2 is being repeatedly pressed & it slows things down an awful lot.