你的理解是错的

Back door roth conversion uses pro-rata when converting to Roth.

For example, if you have $6000 after tax traditional IRA money and $12000 before tax traditional IRA, no matter how many accounts they are in, they are considered as a whole when converting to Roth.

In this example, whatever money you convert, only 1/3 of that does not need pay additional tax, the other 2/3 must pay income tax.