Hi Emma,
Advice on working out net pay can be found here Personal Allowances:
Personal Allowances: adjusted net income
Overtime will be included as this is taxable pay.
As you have mentioned the £50,000 threshold I am assuming you are considering if you need to pay the Higher Income Child Benefit Charge.
I contacted our Child Benefit team and this was their advice:
We can only point you to the guidance on line and the calculator to input income and establish if the charge is payable we can’t make this decision for you.
For Child Benefit you need to opt in or out.
If they opt in and a Charge is payable you will need to do a Self Assessment tax return.
If you need to know more about Allowable deductions you will need to review the guidance here:
High Income Child Benefit Charge
Here is the guidance and calculaor link:
How the charge works
Thank you.