When you use the "Calculated rate of change" functionality on a custom metric, the results come back in fractional values and you should be able to alert on these values. In my case, I want to monitor number of orders for the last 10 minutes. There are always relatively few orders at night and many during the day, but if order flow drops off drastically in a short period of time, I want to know about it.

Unfortunately, I can't just alert when the rate of change drops by 25% (-.25) because alerts can only happen on integer values. 100% drop is my only option and that would mean that orders stopped completely. In which case, the rate of change is a useless option since I could have just alerted that there were 0 orders in the last 10 minutes.