To get to where you are, you had to first answer that the taxable amount on your form is not your taxable amount. Since you marked that you don't feel that is right, then you need to locate your plan information and records.
When did the annuity start (usually with retirement)?- locate the date
How much did you contribute to the plan? - how much did you put in yourself?
Each year, a part of your contribution is not taxable, usually in box 9b - which is why not all of yours is marked as taxable- but since you want to use another method, you need to go through all your returns and determine how much you marked as nontaxable in the other years and enter the total previously recovered.
If you feel that the difference is from insurance or contributions, go back a screen and mark that YES this is the taxable amount in the taxable box.