I agree with pretty much everyone here, LLucido; this is not a good idea. Though your theory is probably sound, the practice of terrorizing someone just to raise an emotion to associate with a RC seems to be overkill defined. Also, unless you're interested in torturing your subject, it is truly a desperate move that could only be used once or twice. Also, scaring people right before bedtime is a fine way to induce nightmares; becoming lucid from such unpleasantness doesn't seem a fair trade to me.

On top of all that, there is also an excellent chance that your success was coincidental, or the true emotion at play here was based more in your girlfriend's interest in becoming lucid than in your attempt to scare her.

And on top of all that, yes, one-time success does equal 100%, but not really: to prove something is effective you need to repeat it successfully many times, and I highly recommend that you do not go there.

tl;dr: this approach might be theoretically sound, but it is likely not a good idea.