How to Find Your Financial Independence Number

Jeff Yang: My question is regarding how someone should try to find their financial independence number when they are on the younger side. I am 47 and have two kids, so my expenses are very high for things like private school and big family vacations. When trying to use the 4% rule, it is so hard to know how much my expenses will be at say age 75 with social security coming in and the kids grown up, etc. Just curious if you have any methodology you use to help figure this number out.