I like to think of this problem as you being the designer for your own personal vehicle. You want it to be all things: safe, high mpg, good performance, etc.
However the labs who test your materials are not very good so their test results vary considerably from day to day even if the product does not change. What do you do? You can be very conservative, be a worst-case designer, and stress safety. However you may find then that although you build the car from battleship steel, the cost and weight are prohibitive and the mpg are terrible. You might prioritize mpg and end up with a small light vehicle that get triple digit mpg and doesn't cost very much but depends on luck for protecting your family. Or you might compromise with a design in the middle with properties also in the middle with features like seat belts, air bags, etc.
The analog in your world for the battleship steel is that you design for 100% no matter what. That requirement may mean that you never retire. Or you may opt for the bicycle designing for 95% when the market is at its all time high. Or perhaps something in between designing for 95-99% when the market is at some "midpt " of its long term trend....I don't know how you really know that but you can surely have some idea when you might want to be conservative on your valuation............e.g. check here and set the time scale on the bottom to MAX
S&P 500 Index Chart - Yahoo! Finance
.....or you can wait for W2R's Wheeeee!
For the seat belts and air bags, you might consider what your sources of semi-guaranteed fixed income (perhaps add more?) will be that will cover at least your base expenses so that you don't panic or deplete your portfolio excessively when the market tanks. In the end, no magic number ........just the realization as others have said that this is a fuzzy business, nothing is for sure, and you need planning and contingency planning for the journey. Since it is your personal vehicle , nobody will care more about the outcome than you.