This is a very clear grading rubric to use with students' friendly letters. The rubric makes it very easy to grade letters and makes it easy for students to understand what mistakes they made.
Each section of a friendly letter is addressed and the points vary from 4 points to 1 point per section. After grading each section, add the points together for the student's final grade out of 20 possible points.
To find a percentage score take the number of points the student received divided by the total possible points (20) to get the decimal and move the decimal to the right to places to find the percentage.