valuation of a floor benefit serp for income tax purposes under a 457f plan

[Guest rod]

i am trying to determine a method to value a serp benefit subject to a floor. this is a plan under 457f such that a minimum benefit will be paid each month to the retiree. however, the benefit is subject to upwartd adjustment depending upon the investment performance of the investment media. the benfit is reset at the beginning of each year following retirement. we are trying to determine how much to plan for in terms of income tax liability at time of vesting.