The CPU never stops running. If it did the phone would be physically off. So your question can only be answered with the solution I gave you. Because when the screen is on the OS changes state from Sleep/Standby to Awake. And Awake is what is measured by the OS. My original answer also took into account that the phone can become Awake momentarily, without turning the screen on, to deal with things like events (IE E-Mail, SMS, MMS, Apps, Phone Call, Voice Mail, etc). So the Usage Time is around 95% or better of Screen On time with the remaining being CPU on time for event handling.
If you want a yard stick to remove CPU time that isn't related to Screen On time, remove 1 minute per hour based on casual observation.