If you what it to be perfect you can always divide the unit rise by the hip rafters unit length and then using the tan^-1 function find the angle that corresponds to the quotient. For a 12/12 roof that would be tan^-1(12 / 20.78) or tan^-1(.5774) or 30°. You can get the unit length right off the square.

You can also find it by using tan^-1(sin(hip angle)). Knowing that the hip angle of a 12/12 roof is 35.26° this would work out to tan^-1(sin(35.26)) = 30.00°.