Most charting packages have an ATR function. People tend to use a multiple from memory about 1.5 to 2.5 times the ATR are common. Katsz discusses this at some length in The Encylopedia of Trading Systems, probably available from a Library. Chandelier stops are quite highly thought of I believe. You could use the last SH SL (swing high swing low) to hide your stop behind. Or even simpler just use the last 1 2 or 3 bars low. Or 2 consecutive down bars or or or
Anyway hopefully there's a couple of useful ideas in there for further research.