The successful fit of a sweater can often be defined by whether the sweater drapes or droops. The best fit is accomplished when the shoulder shaping of the garment matches the shoulder shaping of the wearer. First you need to measure the shoulder slope. You need at least three hands and one helper for this project. First, place a ruler horizontally across the back at the base of the neck, then place a yard stick horizontally across the back from shoulder to shoulder. Measure the vertical distance between these two horizontal lines. The greater the distance, the more shaping is required at the shoulder and vice versa.
If you are fortunate enough to have a pattern with a diagram, it should be easy to determine what the pattern is allowing for shaping; if not, you will have to dig the info out of the pattern. Look for the length of sweater at the point beginning the shoulder bind offs (if any) and the finished length of the piece. The difference between these two measurements is the pattern allowance for shoulder shaping.
If you have very straight shoulders, you might want to just keep knitting to the full length of the piece and omit the shoulder bind offs. If you have very slanted shoulders, you might need to start binding off ½” to 1″ before the pattern directions. Then you need to put on your math hat. If the pattern calls for a 1″ slope by binding off 6 stitches at the beginning of the next two right side rows and you need a 2″ slope, you can start binding off 1″ sooner and bind off 3 stitches at the beginning of the next four right side rows.