I'm not virangelus, but I do the same thing. I guestimate how long I expect a story to take. I also outline, so that gives me a good idea how many words a story might end up at. For each major plot/story point, I figure I'll have a scene. Each of my scenes range between 1,000 to 2,500 words, more or less depending on how much description I use. I add up the scenes and times that by an average. It doesn't always work, but it gives me something to shoot for.
Out of curiosity, virangelus, how do you know it's 20%?