For a back door IRA conversion, what year does my balance have to be zero?

[Spouse1234]

My employer allows me to roll my Traditional IRA into my 401(k) while I am currently employed with them. I want to do this so I can achieve a zero balance in my Traditional IRA and therefore enjoy the full tax advantage of a back-door Roth IRA conversion. The question is, can I do it for 2022, or do I have to wait until 2023?

If I roll over my Traditional IRA (held by Fidelity) to my employer's 401(k) (also held by Fidelity) in the next few weeks (November 2022), it means I can achieve a zero balance on my Traditional IRA by Dec 31, 2022. If I make a non-deductible contribution to my Traditional IRA for 2022 sometime after that (Dec 2022 to March 2023), and then convert it the next day to a Roth IRA, will the full amount be tax-free, or will it be considered mingled with the amount that was in there for most of 2022? Or should I hold off making the non-deductible contribution until it is for year 2023?

[Critter-3]

it means I can achieve a zero balance on my Traditional IRA by Dec 31, 2022   Bingo ... roth conversion in 2022 needs a zero balance by the end of 2022.