Really great question. I have been recently struggling with fusibles. The directions say use medium heat -what does that mean?
Then when I tried to research why the applique wasn't sticking, it was 1) iron too hot, 2) iron too cold, 3) didn't hold iron on long enough, or 4) iron was held down too long. Arghhhh!
What I plan to do is measure the temperature as Dolphyngyrl showed and start experimenting.