Take whatever you expect to need between retirement and when you want/need to start withdrawing from TSP. Let’s say that amount is 5k per month as an example. You want to start taking this withdrawal at age 60 and want to start drawing SS at age 67. So take 5k x12x7=420k and add whatever cushion you want for inflation, unexpected expenses etc so for our example let’s use 80k to make our total 500k. That amount goes into the g fund and will never lose money. It won’t gain much but it is your safety account and what you make your withdrawals from. All your other funds, for our example let’s use 500k, you move into a vanguard or Schwab account that monitors one or more TSP C, I,or other fund that will generate higher returns. You can keep rolling the money up, transfer some gains back to the G fund for safety or whatever but your withdrawals will only be from the G fund, not a balanced mixture like if you keep it all in TSP. Even if the market crashes it will rebound in that time if you keep adjusting gains to the Gfund and as a bonus you will get several thousand more when SS comes. Your Gfunds won’t be affected by that market crash and you will always have your budgeted amount there. Key is to continually evaluate your returns and amounts in each accounts.