Many people struggle if it comes to user defined thresholds in Redline, but it's not that complicated. Look here and find out how easy it is:

How to create a new threshold:

Assume we need a threshold which defines that every server has at least 50 MB of free memory available. If not, send an email to admin@mydomain.com.

1.Click on Configure / Thresholds / Host.
2.Check the Status Enabled checkbox.
3.Give the new threshold a name, for example: Minimum Host memory.
4.Click on the „Browse“ button and click on the column title „Description“ to sort the table based on that column.
5.Click on the icon for „Memory Free (KB)“.
6.In the „Should be“ line Select „>“ and specify the value 50000. Because this sin't a counter, we do not check the „2 Minutes Delta“ checkbox.
7.Set the severity to „Problem (3).
8.Don't check „Multifire“.
9.Check „Back to Normal“, because it might happen that there are moments when there isn't the required amount of free memory, but it might be a short period of time only. Then the alerts list doesn't list the alert after it's back to normal.
10.Let's say by default all notifications are sent to „gwadmin@mydomain.com“, but only for this threshold we want a notification to „admin@mydomain.com“. In that case specify the email address „admin@mydomain.com“
11.Click „OK to save the new threshold.

That's all. Try it yourself.