Planning vs. Certainty: Don't get Confused

I'm a planner by trade. I love putting together spreadsheets with precise estimates of inflation rates and returns on various asset classes. My spreadsheets generate precise numbers. And the one thing I know for certain about these numbers is that they will be wrong!  So does that mean that doing planning is foolhardy? Not at all. Having a plan in place enables you to course correct when the inevitable bumps and changes along the road of life occur.  The important thing is not to confuse your plan with certainty.  Do you have a financial plan? If so, how have you found it to be valuable in recent years?