The Amortization and Annuitization methods allow the qualified plan balance over your life expectancy (or joint life expectancy of you and your oldest primary beneficiary) using a "reasonable" interest rate assumption for determining earnings. The IRS has defined a reasonable rate of return to be not more than 120% of the federal mid-term rate for either of the two months immediately preceding the distribution. The Sec. 72(t)/(q) Calculator limits the rate to the AFR maximum rate or allows you to specify a rate.
The IRS applicable federal rates (AFR) are updated monthly.