What is the typical percentage of knowledge learned in college that a graduate mechanical engineer need to have for a job?

You should have a general understanding of mechanical engineering as a whole. At the same time, a more detailed knowledge in a specific sector of mechanical engineering is required. If you have a chosen specialty that you feel you are more inclined to lean toward, then definitely do the studying and course work needed to complete the program. There are specific certificate programs that you can obtain in order to prove your ability and competency to an employer.