Time Evaluation for Part Time Employees

Part Time employees OverTime calcualtions are as follows:

Rule:

If employee is works less than 37 hours of work, Overtime above his planned hours in the week should be paid as OT*1.0. If employee works more than 37 hours, above 37 hours should be paid as different based on the day.

Example:

If employee is on Rotating shift (i.e. This week working hours are 18 and next week 27 hours), in infotype 0007 his average working hours showing as 27 hours, but actulally every week his planned hours are 18 hr or 27 hrs.

In time evaluation, i am reading HRS=IWEHRS (to read weekly work hours), but this is not the actula weekhours employee worked.

Can anyone help me in finding the suitable Operation to read his Planned work hours based on His Periodic work schedule.