Percent Rank and Curved Grading in Excel

Percent Rank and Curved Grading in Excel

I am trying to design a gradebook for a class which follows relative
grading. Relative grading here means 30% As, 40% Bs, 30% C and lower. In a
class of 11, for example, 3 students will get As, 5 will get Bs, and 3
will get C or lower.
Students will initially receive a normal percentage grade for total
coursework (0-100% <- I already have this calculated out with proper
functions and formula). But, what I want to know is how can I rank them,
and then based on the number of students and relative grading percentages,
assign their rank the proper grade?
I have already used =PercentRank to rank them with a percentage, but not
sure how to transform that percentage into a curved distribution of
grades.
I have included the grading book I am using, which is based off of
Vertex42's.
https://docs.google.com/spreadsheet/ccc?key=0AqvTp4ajjRSUdDhZT1hFSkRQRXUtQ1AwWnR6bFRUT0E&usp=sharing