Unpaid leave not getting calculated correctly in Payroll

Hi All,

I have a period work schedule where the employee works from Monday to Saturday & Sunday is off (4 Sundays in a month). When I mark one day unpaid leave for the employee & run the payroll for June then the basic salary gets deducted by one day but based on 26 days (Basic Sal/26 days) & not 30 days. The Sundays are not considered which is wrong. Pls help me how the system can calculate per day salary based on 30 days