When you said 20% pre-tax and 20% post tax, you didn't say 20% of what. I assumed you were talking about percentages of the distribution and is the calculation discussed prior to your post. However, it's now apparent that you meant that if your distribution is 20% of the sum of your year-end balance plus distributions and conversions, your distribution consists of 20% of your basis and 20% of your taxable money. That's an alternative way of calculating the nontaxable and taxable amounts, essentially a rearrangement of the order of the mathematical operations, but is not the way it is done on Form 8606. In the case where there is a combination of regular distributions and Roth conversions (for which the nontaxable and taxable amounts of each are required to be determined separately), the way it's done on Form 8606 is simpler, involving only one division instead of two.
