A SERP is also referred to as a Nonqualified Deferred Compensation (NQDC) Plan.
Pension plans, lump sum payments, present value calculations are normally preformed using rates and formula’s provided by the PBGC “Pension Benefit Guaranty Corp” a US Govt Agency which is the agency that insures private pensions of certain employers (ie. Employers that go bankrupt). While the PBGC has many rates and formula’s the most basic formula used is the present value of an annuity formula using the prescribed rates from the PBGC. The current long term rate for lump sum’s is 4%.
The web site for PBGC rates & formula is http://www.pbgc.gov/prac/interest/monthly.html