You can filter the ot lines by the code of the desired rate. For example, below is the code for the OTREG0006

amount = 0.0
for line in payslip.payslip_ot_line_ids.filtered(lambda l: l.overtime_rule_id.code == 'OTREG0006'):
	amount += 20 * line.number_of_hours * line.rate / 100.0
result = amount

actually, the computation is right, but i want to separate all each overtime rate into different salary rule computation for intensive report purpose.

There is a rule named "Overtime Allowance Amount" added by this module, in which the computation is as follow

# find total working hours
total_hours = sum(line.number_of_hours for line in payslip.worked_days_line_ids)

# cost per hour is calculated base on contract's wage and above found total hours
hour_cost = total_hours and contract.wage / total_hours or 0.0

# start calculating overtime cost
amount = 0.0
for line in payslip.payslip_ot_line_ids:
	if payslip.contract_id:
		amount += hour_cost * line.number_of_hours * line.rate / 100
result = amount

• base amount: $20/hour
• rate of the OTREG0006: 150%
• rate of the OTREG0617: 150%
• rate of the OTSUN0006: 200%
• rate of the OTSUN0617: 200%
• the employee has 10 overtime hours against the OTREG0006 and 5 overtime hours against the OTSUN0006 and 7 overtime hours against the OTSUN0617 and

amount = 0.0
for line in payslip.payslip_ot_line_ids:
	amount += 20 * line.number_of_hours * line.rate / 100.0
# after the loop, the cummulative amount should be 780.0
result = amount