Mark,
It seems you would want to create a process that runs on the last day of every month. From the sound of it, the process would add a leave accrual record with a value of .25. When the manager decrements for leave taken, I presume that's a record with a negative 1 value. The general process is pretty simple:
FIND ALL Employee (or whatever segment you need using a WHERE clause)
CREATE LeaveAccrual WITH LeaveAccrual.Employee=Employee,Value=.25