contributions/recharacterizations can be backdated to the prior tax year but the subsequent conversion back to Roth as part of a backdoor process is reported in the calendar year it is done, so next year you will get a 1099-R for the conversion for tax year 2026, which will have a finalized market value for whatever was converted.
this backdoor conversion split across tax years is normal situation when backdating the contribution, referred to in the Backdoor Roth page you referenced:
If you'll receive a 2026 1099-R in 2026, wait to report it on your 2026 taxes. In this case, only complete Step 1 below for your 2025 taxes. You’ll complete the second step next year when filing your taxes for 2026.
so for 2025 taxes - you would report the contribution & recharacterization only, and the end result will be a non-deductible contribution to your Trad IRA (for the original amount of the contribution), reported on Form 8606 Lines 1 & 14 which will carry forward the basis for 2026 when you report the conversion.
when you enter the recharacterization you will provide the market value that was moved back to Trad IRA and an explanation statement, but the market value of the Trad IRA doesn't matter until you convert it (and you will be taxed on any gains thru this whole process). I'm not 100% certain if there is any other issue if the final market value changes a bit but I don't think it affects your 2025 return.
perhaps @DanaB27 or @dmertz can confirm any other issues.
