To manage these settings, click your Account Name and select Settings from the dropdown menu.
Click Research from the left menu.
You can not only select what a Great Rank is to you, but you can do so on the individual category level, as well as change the total items in a category.
The category list is scrollable on the left side.
The fields for the Total Items and A Great Rank Is are editable so you can update those whenever you get new information.
The far-right column automatically calculates the Great Rank number based on the Total and the Rank percentage you enter.