Please see the attached for a possible solution.
In most cases the parameters for a generator cannot be controlled using generator attributes. Those values are interrogated just once before the generator builds. Since the X Angle of the corner is potentially different each time, it won't work. An exception is when a generator is nested. In this case you can use the attributes of the Generator to which the nested array is attached. In this example, I have changed it so that the length of the hips uses that approach.
To get the angle there's no need to use the generator's parameters, instead, you can use a Transform node to rotate the nested array.
I hope that helps,