Took my first RMD from my IRA this year and have cost basis in my IRA. I use Turbo Tax and know how to determine the percentage of my IRA that is not taxable based on my IRA cost basis. But also retired and had to take a RMD distribution from my 401K before rolling it over to my IRA. Do I apply my cost basis percentage to just my IRA or do I apply it to my combined IRA/401K distribution. For example, if I have a $100,000 IRA with a $10,000 cost basis, I know that 10% of my IRA RMD would not be taxable. But if I also had a $900,000 401K, and had to apply my cost basis to both my IRA and 401K, my $10,000 cost basis would only mean 1% of my RMD was non taxable. So, do I apply my cost basis percentage to just my IRA or both my IRA and 401K?
