Hi!
You should create separate events (for example, DISK_SPACE_WARNING, DISK_SPACE_CRITICAL) for each level you wish to have. Then add appropriate thresholds and set your custom events as activation events. For example, for threshold "> 50" set DISK_SPACE_WARNING as event, and for threshold "> 90" set DISK_SPACE_CRITICAL as event. Don't forget to order thresholds properly - in case with disk space, you should start with "> 90", then "> 75", then "> 50". See data collection section in manual for explanation. Add appropriate processing to event processing policy.
Best regards,
Victor