Hello Dave,
I found the below old wily kb:
Question: How does Linux Platform Monitor calculate the CPU utilization?
Answer: Linux Platform Monitor calculates the CPU usage by the following approach.
Utilization %(Process)
It uses getrusage() to get usr and sys CPU time in milliseconds within 1 sec. Then it calculates (usr + sys) / 1000 for the process CPU utilization.
Utilization % (aggregate)
It reads the CPU time for usr, sys, wait, and idle from /proc/stat. Then it calculates (usr + sys) / (usr + sys + wait + idle) for the aggregate CPU utilization.
I hope this helps,
Regars
Sergio