To take it one step further, I don't "try" to keep my hand at any angle. What I try to do is to keep my hand relaxed, except for the hook in my fingers. If my hand is relaxed, I can feel my hand stretch out as I draw the bow, and the back of my hand will automatically be in line with the string. If I try to pull with the muscles in my hand, the back of my hand will not be flat, it will be cupped, and the chances are my fingers will torque the string (put sideways pressure on the string).