If the correct taxable amount is shown in box 2a and box 2b Taxable amount not determined is not marked, you shouldn't be using the Simplified Method to calculate the taxable amount. If you made no after-tax contributions to the pension, the Simplified Method doesn't really need to be used because the entire amount is taxable; zero plan cost means that the entire distribution is taxable. [Edit: You seem to be saying that you have a nonzero amount in box 2a that is different than the amount in box 1, suggesting that the amount in box 2a is the taxable amount that should be included on Form 1040 line 5b.]
The General Rule is not used for calculating the taxable amount of a pension.
