Subject: Re: Partial IRA to Roth Rollover
I thought that the IRS says pre-tax and post-tax amount must be taken ratably, i.e. proportionately. That is certainly how withdrawals are handled.

The back office people at your broker will do what their lawyers & tax experts have deemed to be the right thing. Just like they carry over the basis on taxable stock transfers.

You, a member of the public, will never be able to talk to those people. The CS folks who take phone calls don't know how the handle these unusual things, so take whatever they say with a grain of salt.