If the edge of the quilt top is wavy, I bet it's because of that stack and whack pattern. The edges of your quilt are all on the bias, so it stretches very easily. I would add a border before the prairie points, to square up and stabilize the quilt. Measure your horizontal and vertical lengths across the middle of the quilt rather than on the edges, and then cut your border and pin them together, easing the edge of the quilt top to fit the border. Then you can add your prairie points. Generally, you start at the corners and work inward so the corners match up nicely.
Hope that helps!