Well, it looks like the forum is in a slump, so I suggest you bring this question up again when it becomes more active after the GREs when students start their apps. Those of us still lingering from last year are domestic so we haven't encountered this issue, but my advice would be one thing to international students:
If there's an "offical" formula for converting your school's GPA to the American scale, then use it... But otherwise, do not convert your GPA by any method that does not fairly represent your ability, or your relative position in your class... For instance, strong students in the US usually have GPAs over 3.7ish and top students may have near perfect GPAs (unless they come from a very challenging program or one that's unfriendly with grades)... Many foreign schools make the mean sit at a much lower "percent out of the maximum"... I recall a student from France saying his scale is out of 20 and anything 15 or higher is considered phenomenal... but if you converted 15/20 linearly with the US 4.0 scale, you'd have a 3.0 which wouldn't get you accepted anywhere... even St. Lucifer Institute of Technology.
So in a case like that, I think it's best to leave that part of the application blank. Hopefully the committees have heard of your school. Give your rank in your class (or have one of your recommenders do it). If you're one of the top students in your department, have your recommender reveal the average GPA of the students in your program in their own system and that will clarify your potential way better than some unreliable conversion to the 4.0 scale.
PS- Also, if you convert your GPA in an unofficial way, the school could get suspicious that you're making it up if it looks good.Edit:
Not long after I wrote this post, a token example appeared...
I studied in France for 5 years at INSA Lyon, INSA Toulouse, the University of Toulouse and the Ecole Normale Supérieure de Lyon, I was in the top 10% like you with 15/20 averages and I applied to 9 Universities last year for the PhD : Harvard, Santa Barbara, Boston U, Washington, Miami, Northeastern, Oregon, UPenn, and another one I don't remember I had 109/120 at the Toefl, 900 at the Physics GRE and 800 in Math and 550 in Verbal. I had 2 recommendations letters from Toulouse, where I was in the top 5% and one from Italy, where I did an internship. I got accepted at Miami, Northeastern and Boston U and I chose Northeastern because they are one of the best in biophysics (they even made the cover of Nature in June)...so, from my experience, I would say that it is not so easy to get in the States. Perhaps they are put off by the grades we get in France, 15/20 is converted to 3.0/4 and that is not good for the top Universities... I remember that once I got a 20 at an exam and the guy who was in charge of the Master's lowered it to 16.5 because "20/20 is way too much" while in the US it is not uncommon to get a 4.0 ... good luck, apply to all the schools you want to get into, but be warned not to get your hopes too high ...