You don't have to, the TurboTax program can calculate that for you.
Otherwise, the process you would use to do it depends on when your father started to draw the pension.
"Annuity starting date in 1998 or later.
If you made annuity payments from a qualified plan under section 401(a), 403(a), or 403(b) and the annuity starting date is in 1998 or later, you must use the simplified method under section 72(d)(1) to figure the taxable amount. Under this method, the expected number of payments you use to figure the taxable amount depends on whether the payments are based on the life of one or more than one person. See Notice 98-2, 1998-1 C.B. 266, and Pub. 575, Pension and Annuity Income, to help you figure the taxable amount to enter in box 2a.
Annuity starting date after November 18, 1996, and before 1998.
Under the simplified method for figuring the taxable amount, the expected number of payments is based only on the primary annuitant's age on the annuity starting date. See Notice 98-2.
Annuity starting date before November 19, 1996.
If you properly used the rules in effect before November 19, 1996, for annuities that started before that date, continue to report using those rules. No changes are necessary."
LINK to Simplified Worksheet
IRS 1099-R
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