How do I pull standard working hours in to voluntary deduction formula?

Summary: We have a requirement to create a voluntary deduction element which deducts one hours pay. We are trying to create a formula which performs the following calculation, FTE salary/52/standard working hours. We have not been able to pull standard working hours in to the formula (global payroll). We created payroll access to hr type formula to fetch assignment details in to our deduction formula. This snippet is working for earnings element formula with the same type as global payroll but it's not working for deductions formula (it's not getting details from assignment screen). Any suggestions on how to achieve the