Lock in Monthly Goal when sending report
At the moment the Monthly Goal defaults to what is defined in Settings > Schedule, and can be overridden at a month level in the Activity > Hours screen.
However if you edit the default value in the Schedule, then it will change the goal set for previous months that did not have an "overridden" value.
The idea is to store the goal set for each month when the report is "sent". This means that any future change to the default setting will not confuse what the goal was for previous months.