I like to be alerted when an item reaches a new historical low price. However, sometimes it would be good to have the alert trigger if the price was within a margin higher than the historical low, for example 5% higher. This margin could be determined and set by the user within the alert editing page. By allowing this margin, it would mean that an item could alert if it was only a few pence/cents higher than the historical low (or whatever the margin worked out to be), I'd still get an alert.