Another way to set trailing stops is to use a stop based on volatility, rather than a %. The general consensus among pro traders is that volatility stops work better than % because they are adaptive to current market conditions.
AverageTrueRange stops are one simple way to do it.
Here is some info on ATR from Chuck LeBeau's site:
Link
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How to calculate Average True Range (ATR).
Range: This is simply the difference between the high point and the low point of any bar.
True Range: This is the GREATEST of the following:
1. The distance from today's high to today's low
2. The distance from yesterday's close to today's high, or
3. The distance from yesterday's close to today's low
True range is different from range whenever there is a gap in prices from one bar to the next.
Average True Range is simply the true range averaged over a number of bars of data.
To make ATR adaptive to recent changes in volatility, use a short average (2 to 10 bars). To make the ATR reflective of "normal" volatility use 20 to 50 bars or more.
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One way to use ATR is to "hang" the ATR distance from the Highest high achieved during your trade (for long trades). LeBeau calls this a 'chandelier' stop.