In your example you have 5 anomoly minutes out of 6 minutes total, for a ratio of 83.3%. It does overstate the number of anomoly minutes because of the way you round the start and stop times to the nearest minute and then throw in an exrtra minute. The alarm might have actually been anywhere from 3 to 5 minutes long. For example, if the alarm came on at 9:02:29 you round down to 9:02; and if it goes off at 9:05:31 you round up to 9:06. So the actual time of the alarm may be only 3:02, yet you've rounded it all the way up to 5. The longest it could have possibly been is from 9:01:31 to 9:06:29, or 4:58 (just shy of 5 minutes). As long as you collect data on a lot of alarms the rounding will wash out, but you shouldn't be adding that extra minute. That way this alarm would be reported as 4 minutes, not 5, which is the average of the extremes.