The Application Process

I know this is an oft repeated concern of many Indian students but how do we (or do we at all) convert our Indian percentage scores to GPA? Some of our institutions do not give very high scores in certain streams - for instance, the person who topped my batch in Engineering here from my college here in India got a 75% while this may not be construed as a high score if we were to convert this into a US GPA. I've heard that some institutes do not insist on GPAs if they are not available and instead want to look at your original transcripts. But if, for instance, this guy who topped class and got 75% sends his scores, how would anyone know that this was a good score? Would the application essay help here in explaining this issue? Would really appreciate some thoughts on this.