Accrual should prorate for newly joined employee by using Simple Balance Multiplier Formula

Dear Gurus,

In Absence setup, we use the Simple_Balance_Mutiplier Fast Formula, as the standard feature of this formula every employee who is joining in the organization except first day of the pay period he/she is not entitled to get the leave accrual, but we want to change it to pro rata basis for the first period of the employee joining month, can somebody help me where to change this in the given Fast Formula.

Regards,