5 ways to find a stop loss level:

1) Fixed percentage: 5-10% is enough
2) Breakout level: 3-5% below the breakout level.
3) Moving averages: Below the 10, 20 or 50 MA.
4) Trendline: A multiple tested trendline is a good place.
5) Average True Range: Use 2 or 3 times the ATR.