For iPhones, I always uses 80 C, but I work with people who use 99. It really depends on how long you want to leave the phone on the mat. If you use a lower temperature, you can leave it on for longer without damaging any internal components with the heat, but higher temperatures give you more time to work with after you take it off the mat. It’s up to personal preference, but 80 C is close to the minimum. You may be able to get away with 70, but you’d risk damaging the internals by having to leave it on the heat for so long.