According to the Pune University rules, the conversion of SGPA to percentage is done in the following way. 

Percentage of marks(%) = CGPA * 8.80 

Where, CGPA = \((\sum_{i=0}^{p} C_i G_i) /(\sum_{i=0}^{p} C_i)\)

CGPA = [\(\sum\)Grade points earned * Credits for each course] / Total Credits

Hope this clears your doubts. 

I have a few friends from Pune University and they were able to provide some insights regarding this. According to them, in order to convert your SGPA/CGPA to a percentage, you need to multiply your grade by 8.8.

Let’s say your SGPA is 9.0. Then your percentage will be 9.0*8.80 = 79.29%. 

Hope this helps you understand the process.