If like me you have trouble finding that 30 degree angle (and have an Android device) I've just found an angle measurer for free on the Play Store, the one I'm using is called "Leveler (level tool)" but there is loads of others. It uses your camera to measure the angle of an object by putting a red line on the screen with the degrees so no more guessing, and once I get used to what it looks like I probably won't need it.
Hope this was of some use to someone.
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