Is that the inflation that excludes food & fuel?
Anyway, I'm not so sure the usual models are in play. You've got the crowding-out effects of massive non-SS transfer payments, future employment & wage projections, etc. Add to that the huge percentage of SS payments are not paid to seniors, fraud, etc.
Probably make more sense to compute present value in terms of a future governed more by DEFLATION than INFLATION ~ much like today