How to get the appliance "Load" percentage seen in the SSL Visibility management GUI console dashboard "Overview" tab:
Via SNMP
The SSLv admin console GUI System load percentage value is not available via SNMP MIB but it is possible to setup SSLv to send an alert email when its value passes a set threshold (default values are: 75% for warning and 90% for Critical).
Please refer to SSLv admin manual "Modify Alert Metric Rules" and "Define Email Destinations for Alerts" chapters