A closed report for the month on active maintenance jobs at the start of the month, then jobs reported, jobs closed and active jobs at the end of that month. This would require tracking dates for custom maintenance filters too, for example, the date that a job moved from "actioned" to "completed awaiting invoice" (Which is a stage we use when we know the job is done but we have not had the invoice yet)
The current maintenance export is very large and doesn't quite allow you to get to the basics of overall productivity in the month, so where you started and ended, due to the fact that you do not know when a job moved into a "custom phase".