I think it is entirely possible to use a curve, or even a scale to adjust a percentage score to a numerical grade. You can establish a minimum acceptable proficiency... say 55%, less than that, you fail. over that, you can have a curve that adjusts against each student and other students in that class (or even prior classes) on the same level/exam for a final grade... accepting 10% will get a D, and 10% will get an A, with the rest spread evenly with a B and C.