Gradebook

The LAE grader report does this though it sounds more like you'd like to be doing this from the assignment grading interface.  LAE Grader uses whatever percentages you've set up in your letter grades for the course so that when you input a 'B-' it uses 1% point down from the next grade up as its calculation (giving the student the 'best' B- ) and calculates the points based on that percentage against the maximum points for the item.