As far as I have researched, you should NEVER convert percentages to the American GPA system.
http://gmatclub.com/forum/how-to-convert-indian-gpa-percentage-to-us-4-pt-gpa-scale-124249.html
That’s actually referring to people applying for a Master’s in the US, but I think it applies to us as well.
Most colleges have their own conversion system, so if you send them your transcripts with percentages/grades, they should convert it to GPA themselves. Hope I helped!