Lesson 3: One stock should never be allowed to decide your year. Here is the math behind why.Which portfolio carries more single-stock risk?Go with your gut.Two 12-stock portfolios. In one, the biggest holding is 8%. In the other, it's 38%.AAbout the sameBoth hold twelve stocks.BThe 8% oneEven sizing is its own problem.CThe 38% oneOne name can sink the year.