Decomp tube correct angle

[desert rooster cruiser]

Not sure if this question has ever been asked but I recently found out there is supposed to be a set angle of 45 degree on these decomp tube's. So my question is how are you to calculate the angle and what are you using as a base line. It all makes sense being the grates have a angle to them so that it directs the def to the SCR. I will try and post a picture of mine so we can try and figure this out. Thanks everyone.