There are some rules that I follow to earn a legendary (currently I got 3 and only spend 20USD in game)
First: Alway watch all possible place. When you about to enter roll, just let it go slowly don’t push button yet. There are 4 slots contain legendary in a single wheel.
Second: Try to remmember somewhere special in that wheel. For example: 3 super pic in a row -> 2 normal -> 1 legendary / 1 super epic -> 1 normal -> 1 super epic -> 1 normal -> legendary … Or some kind like that.
Third: There are 70% that’ you will get monster next to where you push stop and 30% is 2 slots next to where you push that button.
Forth: Alway wait for a drop rate increase event.
Sorry for my bad english