That's only one variable of many. The spill could occur for longer than 100 days, or shorter (likely longer). The spill rate could be higher than 60K barrels per day, or lower (who knows). The breach in the well bore could be causing an even greater spill than's being evaluated at the well head.
The problem is that no one knows. The risk is too great to assume BP survives. The only credible long strategy is to assume the US arm of BP goes BK and the rest of the business survives. I don't know how credible this is. I do know that if BP goes this route, their business will be vary impared outside the US because other Country's likely won't want to do business with a company that doesn't belly up to their obligations. Particularly one that has such weak internal control and risk management proceedures.