In the limit, you want this to approach 1 (namely, you complete more or less everything you commit). A bad situation is where you're overcommitted and have many committed things, but few completed ones (e.g. 10/2), which results in a high ratio. But, with your proposed metric, you'd achieve the asymptotically ideal ratio with 2 committed projects and only 1 completed one, and in fact the global optimum is "I promised nothing and did nothing, yet achieved a ratio of 50%".