The question is a little confusing, which I think is why you're not getting any responses. I hope this helps, I'm no pro!
Tax basis isn't important for an IRA, since you don't report capital gains on sales within an IRA or on withdrawals from an IRA (the dollar amount withdrawn is reported as income). Are you assuming you can only roll over your "basis" from the IRA to the Roth IRA? If so, my understanding is that you can roll over the entire amount, no need to consider your "basis".
You will need to wait 5 years to withdraw the funds from the Roth IRA, or face penalty This is where your contribution amount or "basis" is important, since you will be able to withdraw your rollover amount but you won't be able to withdraw the earnings (without paying penalty).
I don't see any benefit from rolling your IRA to your 403b.
Are you going to take another job, or are you going to be retiring? If you are retiring, consider waiting until next year to roll your funds from the IRA to the Roth IRA. That rollover is reported as income on your tax return, so doing the rollover during a lower income year is usually a good idea.
I plan to leave my job later this year. I have a 403(b) with my current employer that allows rollovers from a traditional IRA. I also have a traditional IRA with an after tax cost basis of $27K. I'd like to isolate that tax basis so that I can convert it tax free to my Roth IRA, enabling me to withdraw it penalty free if needed. The 403(b), traditional IRA and Roth IRA accounts are all with Vanguard.
This is what I'm currently considering:
- convert $27K within my traditional IRA to the Prime Money Market fund to keep it separate
- rollover the balance (minus that $27K) of my traditional IRA to my 403(b)
- convert the remaining $27K balance of my traditional IRA into my Roth IRA
- once I leave my job, roll my 403(b) back to my traditional IRA
Does this approach make sense?