It seems to me like this question should (has?) to be answered by the plan document, but if the plan document seems to be silent:

How is life expectancy calculated for purposes of determining the period for installments in a defined contribution (DC) plan? In many DC plans, I see one of the distribution options as: "installment payments over a fixed term not to exceed the life expectancy of the participant or the joint lives of the participant and beneficiary. However, this is the first time I can remember a participant inquiring about this option. Is the life expectancy always determined under IRS Reg. 1.72-9 Tables V and VI? Is there a choice based on IRS Notice 89-25 Q&A 12?

The participant in this case is about 50, so I am not asking about a required minimum distribution situation.

Also, are ages based on Age Last Birthday or Age Nearest Birthday? Is any type of interpolation necessary?

If the plan document is silent on this issue, is it a plan defect? Or could I just refer to the rules for required minimum distributions in the document and use the life expectancy definition from that section?

Thanks for help with any part of this question.

I believe that life expectancy pretty much has to be determined by tables V & VI. Notice 89-25 gives some lattitude in mortality tables if using the annuity factor method, but the real difference in the amount of the annuity comes from varying interest rate assumptions.

According to a notation in Tax Facts, the tables are entered with the age of the annuitant at their birthday nearest the annuity starting date. No interpolation is necessary (with the dread exception of determining a non-spouse's life expectancy when using joint lives).

Hope this is of some help.