How to convert from one grading scheme to another?
There is no clear answer for conversion between grading schemes in different countries. Sure, you could numerically try to convert using ratios and proportions as you are currently tying to do but they don't really mean anything because of the following two salient reasons:


Grade Inflation and Deflation: Grades mean differently in different institutions across different disciplines and in different courses. For instance, certain highly ranked universities in the USA are very well known for grade inflation. A 3.5 overall GPA is what almost everyone gets. On the other hand, certain other highly ranked universities suffer from grade inflation where getting a 3.0 GPA in certain courses and in certain majors is very different. Hence, merely using some base metric to convert between grading schemes of different countries is not a very good idea.
Differential grading within the same country: Not every country has a standardized grading system. For instance, India has at least 5 different grading schemes that I am aware of including but not limited to absolute CGPA's on a 4.0 scale, a percentage system on 100 and a relative CGPA system on a 10.0 scale.


tl;dr

Don't convert between different grading schemes unless it is explicitly mentioned what minimum grades you need for your own country. Just apply and hope for the best.