Self assessment questions

Hi. I have submited my tax return for last year, and had added expenses relating to work as I work from home (£312) plus professional subscriptions (~£100) and i've looked at my tax code for this year, and see that the amount is already on my tax code as £414. Does that mean it was probably on a tax code least year, and in which case i've effectively double claimed for it? Or should it still be put on my tax return, and the total adjusted income vs. tax paid at year-end is really what matters in terms of actual figures? Second question, according to my 2022-23 tax return I overpaid by around £900 - does that come back to me as a refund and if so when and how can I expect it? Third question, i'm really struggling to figure out how HMRC calculates reduced personal allowance for over 100-125 threshold, as I know it reduces by £1 for every £2 earned, and in theory then £110,000 = (110000-100000)/2 = £5000 but applying that formula doesn't get me the figure that HMRC displays on the "view your calculation" page. Is there a way to see the workings out? Thanks

The third question was answered already in a separate question. Thank you in advance and anticipation of the first two questions being answered however.