Calculating Item Discrimination Index for a Likert Questionnaire

I am currently in the early stages of developing a scale that measures prejudicial attitudes. I have just run a pilot study with the first draft of the questionnaire, and I am now in the process of trying to narrow it down to a smaller number of questions.

Right now, I am trying to do item analysis by calculating the item discrimination index in Microsoft Excel. I am rather unfamiliar with the process since I was only ever taught how to do it using dichotomous questions with definitive right or wrong answers. Since I am using a Likert scale ranging from 1 to 7 should I use the item's average to categorize participants' responses so that those who score at or above the average are considered "correct" and those who score below are "incorrect". Thank you for your time in advance.