Short Term Disability

I am new to this site. Our company is currently setting up the Lawson system for benefits/payroll. Our short term disability benefit premium is calculated as follows:

annual salary / 52 = weekly salary
weekly salary * 60% = weekly benefit
Weekly benefit /10 = calculation amount
Calculation amount * .77 = monthly premium

the calculations in Lawson are based on annual salary. Is there a way to get the premium to calculate based on the weekly benefit? Or should I use one of the benefit salary fields, and put the calculated benefit amount in there each time I add a person?

thanks for any feedback that anyone has.