Hi Dilk ,

 contract.wage/30/contract.resource_calendar_id.hours_per_day to calculate the amount per hour

employee.total_overtime to get total overtime not within one month

to calculate overtime in payroll for a specific period, try this code:

overtime = 0
for line in employee.overtime_ids:
if payslip.date_from <= line.date <= payslip.date_to:
​overtime+= line.duration
result = (contract.wage/30/contract.resource_calendar_id.hours_per_day)*(overtime)

Hope this will help :)