depends on the flashlight type, if you are using the best (purple) you should aim around the neck and just after the killer put the survivor on his shoulder. if its a green or yellow flashlight you can aim to the face but start blinding him before the killer put the survivor on his shoulder. At least this is how it works for me, even sometimes don't work but definitely most of the time. try to get the best angle possible because if you are trying to flashlight the killer from a 90° angle it will take more time to blind the killer so you will have to time it even more precisely. Hope this helps you, just keep doing it you'll get it.