Under PPA, the lump sum limit seems to be defined in terms of the interest rate (ie. the greater of 5.5%, the rate that would provide 105% of the benefit that would be provided using the s417(e) rate, or the rate in the plan.

What if your plan actuarial equivalence is 6% and the 1983GAM table (50% blend)? Is the max lump sum based on 6% (being the highest rate of the 3) and the plan mortality table (83GAM(50%))? Or, 6% and the latest GATT (ie applicable mortality table)?

I would say the plan table of GAM83 (blended).

In general I would say to use the plan table, as long as the lump sum amount does not exceed 415 based on the statutory 5.5% and applicable table.

That's the IRS's position. But, the code says use the greater of plan interest rate or 5.5% and says nothing about considering plan's mortality. So I don't know how the IRS justify their position.