A spreadsheet is a good place to start to get a feel for how the numbers affect each other, but it won't give you a very good picture of what's likely to happen.
One big problem, as previous poster has mentioned, is that averages don't account for what really happens. Suppose you make a 2% inflation assumption; you won't get an even 2% year after year. Instead, it might be more like 4%, 4%, 1%, 1%, 0% or 0%, 1%, 1%, 4%, 4% and it makes a difference in which order things happen. High inflation early in retirement means that you'll have to spend more money than if it occurs late in retirement.
One way around this problem is to use a large number of different sequences of return to get an overall picture. That's what Firecalc http://www.firecalc.com/
or Flexible Retirement Planner http://www.flexibleretirementplanner.com/wp/
do. Fidelity also offers a retirement planner. Any one of them will give you a more complete picture than your spreadsheet.