How do I cut the dovetail at the correct angle

Get A Speed Square, First Is This A Rafter Or You Making Dovetail Joints, If It's a Dovetail Joint You Make The Angle What You Want, Say 22.5 Just Make The Other Joints The Same To Make It Fit Together.
If It's a Rafter Your Just Going To Have To Buy You A Square And Read Up On It. Its Not Complicated But With Out Pics To Show You I Couldn't Explain It
Hope This Helps , Nichols Trim Carpentry

If Your Pitch Is A 4 And 12 The End Rafter Cut Will Be The Long Cut Of A 4 And 12. Usually I Come Up An 1 1/2 And 3 1/2 Over To Fit The Stud Wall