I went home last weekend, and was able to figure out what to do next on the sweater. I was wrong - I didn't get the pattern from Elizabeth Zimmerman, it was from Knitting in the Old Way, by Priscilla Gibson-Roberts. It uses the percentage system, but gives more exact directions for different shapes and styles. I chose the "fitted sweater", which is a modification of the "fitted vest". I added a fitted waist and a square neck, with a steek so I can cut it up the front.
The armhole shaping turned out to be very easy - cast off one stitch on either side of the front every other round. The neck shaping consisted of casting off 20 stitches on either side of the steek, it will be about 8 inches wide. It has sloped shoulders, which means a short turn row or two at the top of the shoulders. I'm not sure how many, I'll have to look at the two sides together once I've finished the back. I'm working the front above the armholes back and forth, because since it's plain there's no reason not to. I've already finished the front - on to the back! Maybe I'll get to the sleeves before next week.