I recommend following the manufacturer’s heat settings for both your phone case heat press and your phone case blanks. Find the midpoint between the two and start there. 
Keep in mind: different brands often have different temperature and time requirements, so it’s highly recommended to test a few samples before finalizing your project. I know it sounds material/time-consuming, but experimenting is the key to getting great results. 
Here’s a link below—just in case you haven’t checked it out yet. It might help you dial in the right heat settings.