The basic grades and their equivalent percentages are determined by the Programme Executive Committee. They are more or less the same across programs (PGP, IPM, etc).
The formula for converting CGPA to a percentage:
if you have a GPA of 3.33, it would be equivalent to the percentage of 83.33+{(3.33–3.3)/(3.4–3.3)}*(86.67–83.33).