Based on the dashboard view that you have chosen, that is, Last week, Last Month, or a Custom Date Range, the metrics change as per the calculation stated above.
If a custom range view is chosen, the percentages are calculated the same way keeping in mind the window of the time period selected by the user (in days) is considered as the window of past metric comparison.
That is if the custom date range chosen is between 14th Feb - 17th Feb, then the time period selected here would be 4 days. So, this will be compared against the volume of incidents that have come in, in the past 4 days, that is, 10th Feb - 13th Feb.
The calculations would be as follows:\