I believe that NetWorkDays function is not your friend in this scenario.
You need to work on script to calculated number of days per month similar to NetWorkDays.... But better if you can provide an excel file with proper End Dates along with some sample showing your selection and result you expecting.
As a work around could you create a master calendar and add NetworkDays as a field in it. You could populate the values in the load or in the data source for your calendar. So when you select a month you sum the network days of the month selected.