That is not the correct amount.
The amount of the deduction is calculated by subtracting the present value of the lifetime payments that will be made to the beneficiaries from the total amount of the original contribution. Present value is based on IRS actuarial tables that determine life expectancy and assumed earnings by taking into account the donation amount and the applicable gift annuity discount rate.
The charity you donated to should be able to assist you with the calculation.
from IRS website
https://www.irs.gov/retirement-plans/actuarial-tables
