I just moderated a comment about amount of yarn needed for a pattern-less garment. This is when I confess that the main reason I still use patterns as starting points is because I have such a hard time doing this. My local yarn shop has little booklets that estimate sweaters for you. If I were making a vest, I might be tempted to use the sweater amount as a starting point, or possibly 20% less, but I get nervous about running out and that's why I often end up with enough to make 2 sweaters out of the same yarn. When I go by there, I'll look at the booklet and tell you all, hopefully, where you can get them online. Of course, you could get them from my LYS, but that might not be very practical for everybody.
There is a math-measure-gauge swatch way to determine this, too. In fact, there are probably many. A quick google search just found me a very well-written article that explains it better than I can, and I am e-mailing the author to see if I can point you all there and/or talk about it on the show.
Hopefully I'll have a show up in the next few days.