​@LAS You can try the formula below which would bring you the right number of work days.

NETWORKDAYS(Month.'Start Date', Month.'End Date'+1, 'Day of Week'.'Working Days'=True)

You may need to +1 to the end of Month.'End Date', since the second date is not taken into account. If you want to include month end date, you may have to +1.

Under the ‘Day of Week’ dimension , you may need add an additional boolean property to specify which days are considered  working days.
 

​@Min Li Bright Point Thanks, it looks like I dont need the =TRUE portion but this worked for me:

NETWORKDAYS(Month.'Start Date', Month.'End Date'+1, 'Day of Week'.Working_Days)