most universities do not encourage you to convert but report your scores as is
(HBS,Stanford,wharton, etc)
Some like columbia ask ur estimate but insist on official transcripts being sent. The transcripts should have the registrar comment on GPA to % mapping
I am from V.j.t.i, mumbai university where marks above 70% are rare
so for columbia I have mapped my 68.5 % to be equivalent to 3.75 GPA
But my transcript very clearly mentions that % cannot by any means be converted to GPA and there is a detailed explanation of class system
get your registrar to do such a write up
If you are from mumbai university then I can forward you a copy of mine online
Please PM me if you nned one