If you want it reliable, have it serviced. Even the "swiss" movements coming from China can crap out if not serviced. Swiss does not mean the Movement comes directly from the factory in Switzerland and was assembled in a clean room as per Swiss specs.
Some time ago, I tried that app too... just for fun. Didn't work at all for me. I think the biggest problem is the mic of the iPhone wired headset. I also own a PC timing software and the microphone that comes with it, is in a totally different league.