Percentage of what?
Alerts attached to a horizontal price level require an absolute value (the price level).
Alerts attached to a trend line use the price level of the trend line at the point trend line is crossed.
What might work is to create a moving average (20-bar, 50-bar, etc), and then in the signals tab of that MA enable "show alert" for when price crosses the MA curve. If you select the 200-bar MA it would only alert you for the largest moves (eg. that reach the 200-bar MA).