CGT query regarding basic/higher tax band

[d000hg]

Realised my obvious mistake was due to forgetting the HR threshold was £50k these days. CG profits must simply sit on top of all other income and then however much of this is past the HR threshold is taxed at 28%. With the HR threshold ~50k, if I 'earn' 40k and then have a £12700 CG profit, that's why I pay 28% on the top ~2700.

Must have been having a brain fart

Guess I need to issue smaller dividends this year.

[d000hg]

We are selling a property exchanging/completing just before the end of the tax year (phew) and my share of the profit is £25k so the taxable gain is £12700.
Originally I mistakenly took this to mean I would pay either 18% or 28% on £12700 depending if I were basic/HR tax rate on my regular income in 2022/23 and while I know that's wrong, however I can't quite figure out how it does work.

I do know that my total income including the CGT taxable gain is used to determine if I'm a HR tax payer, but then how much of the chargeable gain is taxed at 18% and how much at 28%. I've been playing with calculators from HMRC and a simpler one (same answers) from https://taxscouts.com/calculator/capital-gains-tax/ and while how much of my £12700 is charged at each rate varies I don't see by what method.

e.g. I set 'salary' as £40k and property profit as £25k, why do I pay 18% tax rate on £10,270 and 28% on the remaining £2,430? Clearly it's a sliding scale starting at £37570 but how precisely?

As a separate question, do I then get double taxed on this via income tax in the above example... my total income is £65k for the calculation above, for for SATR is it £65k or £40k? I have not taken Ltd dividends to the HR threshold for 2022/23 yet.

(clearly I will ask the accountant but they are of course very busy this time of year and it might take a few days to get a reply)