Dwarf retic percentages?

I'm having a bit of trouble understanding dwarf percentages and how one comes up with it. I understand that a dwarf bred to a normal yields babies that are 50% dwarf. But how do people come up with 67% or 73%, wouldn't it be 25%, 50%, 75% and 100%? Also I've looked around and can't really find any good information on raising, housing, breeding, the whole 9 on dwarfs and super dwarfs. Are their any books, videos, or websites that have solid info that you recommend? I'm looking to get a super dwarf when I return from Japan but I want to make sure I have done all my homework first.

Dwarfs and sd are pretty much just like their normal counter parts in respect to behavior just in a smaller package from what I've read. Sd stay between 5-7' and some dwarfs have been know to hit 11' or bigger. But I'm no expert, this Is all from just reading and remembering.