First self-assessment -- calculated tax does not match PAYE, trying to work out why

Hello, This year my salary was just over 100K and I'm doing self-assessment for the first time. Unfortunately there's a discrepancy between the PAYE that my company has paid and the calculation by HMRC, and I'm trying to work out whether the company is actually wrong or whether I've just filled in the form wrong (which feels more likely to me). HMRC reported the tax owed as 31716, and my understanding of how they arrived at this calculation is below: The tax bands for 2022/23 are: Total income 107140 Income over 100K 7140 Deduction (income over 100K / 2) 3570 Calculating the Reduced Personal Allowance: Personal Allowance 12570 Reduced Personal Allowance (12570 - 3570) 9000 So, then calculating taxable income: Total income 107140 Reduced Personal Allowance 9000 Total taxable income 98140 Then calculating tax due on the taxable income: Income taxable at the basic rate (50270 - 12570) 37700 at 20% = 7540 Taxable at the higher rate (98140 - 37700) 60440 at 40% = 24176 This brings the total tax due to 7540 + 24176 = 31716, so I think that's how HMRC arrived at the figure. The tax that my employer calculated was 29232, which doesn't line up, and I'm struggling to find how they arrived at that figure because I only receive statements of how much tax was paid each month, but not why that amount of tax was paid. The company is pretty big (10K+ employees across a dozen or so countries) so I'm assuming that the most likely cause is that I've given the wrong information to HMRC in my self-assessment, but I'm struggling to see how I could have done so. I had a few questions that I'm hoping someone can help me with: - Could there be some deductions that I've failed to add which might account for the discrepancy? - Is there some way that I can work out how much PAYE should have been paid each month so I can check whether my employer was doing the right thing? - If I pay this now to meet the deadline but continue to investigate, can I retrospectively ask HMRC to refund tax if it turns out that I accidentally filled the form out incorrectly? Thank you so much for any advice and help; I'm sure this gets easier with practice, but as a first timer I'm finding it hard to know whether I've done this correctly or not. Thanks, Will

Hi williamblack,
Your employer will have calulcated the tax due on your income based on the code for the year.
By the sounds of things, you have had the benefit of full personal allowances for the year when not due.
This extra allowances result in tax due at 40%.
You would also need to check what other allowances and/or deductions were in your tax code#.
Thank you.