Just to illustrate my point, I worked out some numbers based on the original post. Using Jill's balance at age 30 ($31,291), I imagined she gained 20% every year for the next ten years, taking her through age 40. After that, I imagined she lost 4% every year until she was 50. That is an average return of 8% over 20 years. Here's how it comes out:
Age Yr. End Value
30 31291
31 37549
32 45059
33 54070
34 64885
35 77862
36 93434
37 112121
38 134545
39 161454
40 193745
41 185995
42 178555
43 171413
44 164557
45 157974
46 151655
47 145589
48 139766
49 134175
50 128808
Compare to the example from your attachment, which shows her at a balance of $145,846 at age 50. So, with the TSP - where we have no guaranteed return from one year to the next - it really makes little sense to think about one's average return throughout their career. It's all about what you are doing now.