The information in this post is old and for the latest information about this cpi/spi to percentage conversion is in the below link:
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AICTE notification on dated:5th March, 2010
The proposed structure for the Degree/Diploma is as follows.
Grade Point Equivalent Percentage
The link for this is available at http://gtu.ac.in/circulars/11June/Grade Point.pdf Here grade point means cpi