I also found this equation, which seems WAY easier. Has anyone used this?

TH400, TH350BOP, 700-R4: Speedo driven gears have from 34 to 45 teeth. Each tooth added lowers the speedo reading about 2.5%. Each tooth removed raises the speedo reading about 2.5%. In our example, we want to lower the speedo reading from 60mph indicated to 52mph indicated. We need to add teeth to lower the reading by spinning the speedo cable slower. Remember, one tooth = 2.5% so we need to add about six teeth. (15.4%/2.5% = 6.2 teeth).