"[If] you take this 4% recalculation of a volatile account balance every year, your spending is as volatile as your portfolio. That kind of becomes untenable in the real world.... So, [Jon Guyton's] idea was to start with a target -- for example, we're going to start at 4% or 5%, and we're going to assume it creeps up over time. We are going to recalculate and check on it every year. And as long as it's relatively close to where we were targeting, we'll hang tight. But if it gets really far off, we are going to start making changes."