Those total resources are there for your spending any way you wish. Sustainability research determines how much each year you can withdraw. This approach’s goal is to sustain your Standard of Individual Living.
The bottom up approach calculates each of your expenses today carried forward into the future. What if those expenses change? How far out into the future do you calculate those expenses? What if you live longer than that?
The top down approach is also supported by the budgeting philosophy described in the book All Your Worth: The Ultimate Lifetime Money Plan. It is much easier to manage your money when you have a broader perspective on how to relate to money.
When you have main categories already covered, you can spend the rest any way you like.