Answer:
For humans, it's best to give a subQ injection at 90 to 45 degrees. Ninety-degrees is best if you have tented 2 inches of skin. Forty-five degrees only if you have tented one inch of skin.
For livestock though, it is best to give subQ shots at a 30 to 45 degree angle relative to the surface of the body after tenting the skin with thumb and forefingers. This is especially true when you have to give a SubQ shot to livestock like horses, cattle, sheep, and goats.