I like the idea, and if you decide to make an app, I'd give people the option that they can choose the % change on their own, not just set it to 5%, especially not now when the btc is so volatile.
That's a very good suggestion. I would like to have that too. But for me, I guess at least an everyday update on the price of bitcoin is enough for me and an email notification of the percentage that I set that bitcoin has reached.