Based on the Grade Point Average to Percentages given in the Faculty of Science and Engineering Calendar in 2011, we use polynomial regression in Excel to get the following equation:

`f(x) = 0.0076*(x**5)-0.1541*(x**4)+1.154*(x**3)-3.9327*(x**2)+11.017*(x)+38.958`

Here’s a quick and easy way to implement this in Python 3:

```
def yorku_percent(cpga):
"""
Calculates grade percentage from CGPA based on Faculty of Science and Engineering Calendar.
"""
if cpga < 0 or cpga > 9:
print("Invalid GPA")
else:
percent = 0.0076*(cpga**5)-0.1541*(cpga**4)+1.154*(cpga**3)-3.9327*(cpga**2)+11.017*(cpga)+38.958
print("A cumulative GPA of " + str(cpga) + " is approximately " + str(int(percent)) + "%.")
```