60-20-20
3-2-1
The rule of 2/3s

Simple rules:
1) The butt should be 60% of the leader, the portion which steps down quickly should be 20% and the tippet should be 20%.
2) Break your butt down into three sections. The heaviest section of the butt should be 1/2 the length (3/6), the next heaviest section should be 1/3 (2/6) and the final section of the butt should be 1/3.

The stepdown portion should be equal lengths

3) Never stepdown by more than 1/3 in diameter. Put another way, the following section shouldn't be less than 2/3s the width of the preceding segment.

Here's my 10'er.

Butt:
3' of 0.22
2' of 0.19"
1' of 0.17"

Taper:
6" of 0.15
6" of 0.13
6" of 0.11
6" of 0.09

From the last segment of 2X, I can go to as small as 5x.

I usually tie a perfection loop at the end of the 0X. If I need to go to 6X on the tippet, I'll loop on a 6" segment of 3X and then tie on some 6X. 6X slightly breaks the 2/3s rule but works fine.