Prior to amending the formula for configuring working hours, you need to create database items for the input values on the elements you use to record regular and overtime hours.
Use the Element window.
In the Name field, search for the name of the element used to record regular or overtime hours.
Click Input Values to go to the Input Values window.
Select Database Item to create a database item for the input value.
Repeat the above steps for each element you use to record hours.