How strict are listed minimum requirements for admission to a graduate degree program?
No direct conversion of grades is possible between various systems. So, you will have to argue your way into their program, by presenting your results in the best light possible and making a convincing case that you would fulfill their requirements if you were in their system. Because in the end, your application will be judged by people, not computer. At least if you make it clear that your system is quite different.

One of the ways to do it is, as you started to do, by comparing percentiles: figure out what their grade requirement equals to in terms of quantile, and start from there. “I am in the top 25% of my school, which is equivalent to a grade of XX (based on these official statistics)”. That sort of reasoning.