The height of bend in your shafts should be determined by the height of the axle, height of the attachment of the shafts to the cart, and the height of your dog at the point of shoulder.
Advantages of having a bend in the shafts include being able to roll the shafts in or out to adjust height and width for the multiple dogs sharing the same cart set up. The bigger the bend, the more leeway you have to adjust. The less bend, the less adjustability but more stability.
The angle of the bend is not important as long as the ending height is the same. I typically stick with 30 degree bends. For questions: email@example.com