I'm guessing that since the start/end times are for an "outage" that you're using Calendar 0 i.e. 24x7. What happens if the Start date/time is in Oct and the End date/time is in Nov?  Which month would you use to calculate percentage?

I'd suggest looking at Query View Advanced Reports, aka XML Reports.  But first!!! Make sure that you or your SQL guru can create a query that will generate what you need for each ticket (elapsed time of ticket; first and last days of the month for the ticket's End Date; Sum of elapsed times for all tickets where End Date is between the first and last days calculated).  SBM QVAR (XML) reports look like a aux table.  They are essentially the SQL you provide running as a subquery.  This has big implications for report performance and what your SQL can get away with.

I was able to create a Query View Advanced Report aka Advanced XML Report that shows the Elapsed Time for one of our Cases from SUBMITDATE to CLOSE_DATE plus the that Elapsed time as a percentage of all Cases that were closed in the same month.  Let me know if you want me to post it here.

It's kind of a work-in-progress because our Cases can take many days and we might get 100 new Cases a day.  This causes integer numeric overflows because the total ET in seconds for all Cases closed in same month is more than SQL Server MAXINT, and SBM XML reports for SBM 11.8 don't support "numeric-integer64" field types .... AFAICT.  I'm experimenting with doing the Elapsed Time calculations in Minutes or Hours.