no problem - if you have desktop turbotax I find mocking up 2025 return in 2024 software will get you in the ballpark on tax with some allowance for different deduction limits and tax tables between 2024/5 etc, and will generate Form 8606 if you mock up 1099-R for the rollovers/conversions. I don't think TT online version has equivalent to do a separate dummy return and get into the form details, but there are probably other online tax calculators you can use.
Re withholding yes this is treated as "timely" by the IRS regardless of when it occurs thru the year, which would remove the need to pay estimated tax assuming the withholding is sufficient to meet that 'safe harbor' amount. Beware the big downside with it is that IRA money is then being used to pay tax, so you are getting less money converted into your Roth, which can generally be disadvantageous given all the hoops we go thru to get money into these accounts to begin with, it's better to pay the tax with 'outside' funds if able (why do a backdoor Roth if you're then sending IRA money back out as withholding).
Another option depending your situation and the amount of tax, you can make additional withholding from W2 income if applicable, which will also be considered "timely" even if it happens after the conversion, but you only have 6 months left to make up that amount and it take a couple of payroll cycles to adjust.
