I don't have a fixed stop loss in terms of value (I do use them), but what I do when considering where to place a stop loss is consider current market volatility (important at the moment), my risk:reward ratio, taken with a view to what my target is and then I consider at what point do I know I'm wrong i.e. is there a level below my trade (if i'm long) that if broken could lead to a speedy descent. I suggest you have a look into money management in more detail, Van Tharp - Trading your way to financial freedom is a great book for this.
Tom
Fade 'n' Scales