下划线/边框的间距 - 设置特定宽度

I am creating a navigation menu in which has a specific active state. Essentially, its an underline, but with greater spacing and a specific width. So i have begun creating this navigation with a border-bottom attribute and a height. This solves the spacing underneath the navigation and the line, however, how can i set the width of the border underneath the writing so it appears like so:I am creating a navigation menu in which has a