Read cpu usage from iRule

Can anyone help me understand what the cpu command is returning?

http://devcentral.f5.com/wiki/default.aspx/iRules/cpu.html

This link says it is the cpu load. Can I read this to be the cpu usage as reported by the GUI? If so, is this number from 1 to 100?

I don't have a busy box to test it on so all I'm receiving now is zero

when HTTP_REQUEST {
log "CPU - [cpu usage 5sec]"  
}

I'm thinking of writing an iRule that will reject packets after cpu usage reaches 90% so the box doesn't reboot or fail over. I'll probably end up doing it with when CLIENT_ACCEPTED hoping that that will be the point the BIG-IP has to process it the least.