Trike frame riser angle

Can anyone tell me how to determine the angle of the frame "riser" in the photo. Is it a mathematical calcuation based on fork angle or something else. Thanks for any help.

Anything you want it to be. I usually make mine 45, simply because cutting the other end square gives you a 45 rake on the front end.

Thanks for the info. I raked my forks @ 47 degrees and cut the riser angle the same.