Lanternpost,
Whew! A couple of different questions to answer here. First, finish the manuscript or not: I say wait until you're finished. From what I've heard from editors, the split is about 80/20, with 80 percent of the ones I've talked to saying that an unfinished manuscript from a new author is an immediate rejection.
If you send off the first few chapters, most editors--and most agents--are going to want to see the entire novel, assuming they like what they see.
Do you need an agent:
I got an offer on ELANTRIS, then went to an agent, just like Scott Card suggests. However, Eldon Thompson (who also just got a big hardback deal with the first book coming out this summer) said that he got an agent first, then went to publishers.
Right now, about half of the publishers in sf/f will look at unagented submissions. (Tor, Daw, and Ace among them.) Half won't. (Dell, Harper, and Bantam being among them, I believe.)
My suggestion is to try both--seek an agent and a publisher at the same time. There are a few agents who don't like finding out an author has already been marketing a book, but the 80/20 seems to fall down on the side of them not caring on this one.
Finally, 10% or more? When Scott wrote that book, ten was the standard. It's gone up to fifteen. I don't know of any major agencies that don't start at fifteen for new authors. That isn't to say you can't get one for ten, but I don't think is as hard-fast a rule as it used to be.