Child Tax Credits question for all you experts!

[Cantreasonwithunreasonable]

It's financial year end and I have my annual income on my latest payslip.
I estimated a salary figure (let's say eg. 24k) to base CTC on.
My actual figure is £23k taxable, £26k Gross/Pensionable.

Which figure do they use? I ended up doing lots of overtime due to Covid during the past year, and I'm trying to work out how much I'll be paying CTC / HMRC back.

I'm new to this (divorced two years ago, went part-time after 17yrs ft).

Thank you.

23k tax credits only use taxable income. They allow 2.5k over. If you estimated 24k you could earn 26.5k taxable with no overpayment.

Thanks.
So to clarify, they base my monthly payments on the salary I earn AFTER I've paid into my pension / NI contributions etc? Not the gross figure on my P60?
I'm not sure what you mean about the £2.5k either?

It's your salary as it appears on your p60 figure. I think you can only deduct £300 for your pension. (You deduct all your pension then take £300 away from pension amount and readd what's left over as other income. They will have your wage out from RTI

I know they use the salary totals, I can wait until they send HMRC the figures, but I like to contact them to stay bang up to date. I don't want to owe them.
Still not clear which total they use -
Gross salary total
Taxable salary total.