The best approach would be to define the warning and critical thresholds for each interface and create a new alert based on the interface being in a a warning or critical state rather than defining a custom property to contain the value.
Step 1. Settings - > Manage Nodes. Change to interfaces and select 1 or more. Edit properties for the interface(s)
2. Modify the Thresholds as required, to use fixed values, custom values or Dynamic baseline thresholds.
3. Create a new alert to "Alert me when an interface warning or critical threshold is reached = True"
Hope that helps,