For me, My thing has 11 Chapters.
10 % = Tut (Chapter 0 ) and Chapter 1
20 % = Chapter 2
30 % = Chapter 3
ETC
There's not much math involved here or such
EDIT : And for the other percent, You look at what you've done.
You covered it all 100%
You got a little, but something that's fun 12 %
Lol that's what i do