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.

In general, yes. If your grade is well under 3.0, you may be wasting your money for the application fee. Your application may be filtered out by some secretary.

However, I would strongly recommend you to use other criteria to determine which schools you want to apply. You want to consider the programs and the faculty the schools have. If they have the program you are interested in and the professors you would like to go with, then go ahead to apply no matter what. You never know.

Another factor is the school you'll graduate from. Many admission commitees are aware of the different standards that different schools use. They understand that 2.9 from school X may be actually better than 3.5 from school Y. If you can afford to the application fee and you really like that school, go ahead try.

Other factors such as TOEFL and GRE scores, recommendation letters and research potentials are sometimes even more important than your GPA.

Good Luck !