Utilization report - Logged in times

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We are working on a custom report extended from the Agent Utilization report. We removed the session start/end time filter and added a filter from the agent performance interval instead - using the start.  Is there a way to fraction out the logged in time so they are included accurately in our interval searches. 

For example - we want to know utilization from interval 6am - 11am, but if the agent did not log out during that time we are getting null values, which then skews the available% and utilization%. 

Any ideas/tips??