Have you tried using the WES - World Education Service - scale (https://www.wes.org/calculator/login.asp
)? You may find this to be more accurate. This was an often referred to resource by my international admissions staff members when I was the director of MBA admissions.
Additionally, every school is going to recalculate your GPA using your original transcripts. If you must put something on the application, an estimate is fine. However, if you can leave it blank because you don't have an accurate way to calculate your GPA to a 4.0 scale, then I recommend that.
15 Years as Director of MBA Admissions - Now Working For the Applicants as the MBA Admissions Coach
Wendy Flynn is the MBA Admissions Coach, providing comprehensive MBA admissions consulting services. As the former Director of MBA Admissions for 15 years at a top-30 MBA pgoram, Wendy holds deep expertise in admissions issues for Full-Time, Executive and Professional (Part-Time) MBA Programs. Through her blog, MBA Expert Insights, she brings the view of the Director of MBA admissions to MBA applicants and others.
MBA Expert Insights Blog: http://www.mbaadmissionscoach.com/blog/
Member of the AIGAC, Association of International Graduate Admissions Counselors