I use a laboratory quality, micro pressure switch to turn off the nitrous system at 16 psi. I have a ton of extra switches if anyone is interested. Not that expensive too.
Why not use the AUX output on your FAST XFi to trigger the relay. Setup your KPA and TPS.. it does it off your 3 bar MAP. And you can make changes to deactivation on the fly pretty quick on the laptop.
HTH