How to convert from one grading scheme to another? I'm trying to figure out the equivalence between grade schemes in different countries.

According to some Google results:


If you are a U.S. citizen, you need a GPA of 3.2+ (it varies, some say 3.2 and some others say 3.6, etc) to apply for a position in the UK (job or university) where the prerequisite is to have a minimum grade of 2:1.
In UK 2:1 is earned if you have a 60-69% of the points. I've read that 2:1 is the same as 67%.
In Spain we use a grading scheme of 10 points. So according to (2), for have a UK 2:1 you need to have 6-6.9 points.
If you are Spanish and have 6.7, when converting it to GPA the result is 1.5.


However, if you take the above points and treat them as a math equation, the final result will be GPA 1.5 = GPA 3.2, which doesn't make any sense.

GPA 3.2 = UK 2:1
UK 2:1 = Spain 6.7
Spain 6.7 = GPA 1.5


Replacing now...

UK 2:1 = GPA 1.5    
GPA 3.2 = GPA 1.5


My question is if someone has a better explanation on converting grades obtained in different countries.
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.