I'm not sure what you mean by Taxable IRA; there is no such term defined. For tax purposes, all of your traditional IRAs are treated in aggregate as if they were all a single traditional IRA with your basis in nondeductible traditional IRA contribution applied to this aggregate. As you indicated, distributions (other than Qualified Charitable Distributions and HSA Funding Distributions) are a proportionate mix of nontaxable basis and taxable amounts, with the taxable amount calculated on Form 8606.
Distributions from Roth IRAs (in aggregate) and 401(k) accounts are treated separately from each other and separately from your traditional IRAs. Contact your 401(k) plan administrator to inquire about any plan-specific distribution limitations that might exist.
