Sorry that I did not see your questions sooner, Kathryn. Hopefully you already have the answers from your LYS. I did not use any formula - just an extra YO on each purl row. I also allowed myself some leeway in mistake-making. Since I knew the row stitch count for each tooth, if I came out with more or fewer stitches, I just did an inc or k2tog to compensate within the tooth.