You use which ever one is correct, which is likely to be the OPM calculation (which is also the one that the IRS will assume is correct without any evidence to support a different amount being taxable). See How to use the Simplified Method in IRS Pub 575. The difference in the number of expected monthly payments is apparently due to differences regarding the ages of those covered by the annuity (Table 1 for a single-life annuity, Table 2 for a multiple-life annuity). A factor of 310 would apply if it's a single-life annuity and the individual is age 56 to 60 or it's a multiple-life annuity with ages totaling 121 to 130 years. A factor of 210 would apply if it's a single-life annuity and the individual is age 66 to 70 or it's a multiple-life annuity with ages totaling 141 years or more.
https://www.irs.gov/pub/irs-pdf/p575.pdf
Since both you an the OPM probably know the ages, the difference is likely that you are calculating based on it being a single-life annuity while the OPM is calculating based on it being a multiple-life annuity.
