Iteration of beam angles

Hi all,

I am beginner of grasshopper and I like to calculate tree like beam structure in karamba by using gh component.. So I designed simple cross section which will help me to calculate for surfaces later. I want to adjust the angles of the beams iteratively.

If the angle of second beam increases , then the angle of third beam should increase and vice versa. But I tried in gh where I dont know the exact expression to evaluate it. I tried various expressions but still the angle difference between two beams is bigger when its done iteratively. Help me with the correct expression or anyother alternative way of doing it.. Waiting for your help guys. 

I have attached d pics of both the model and gh component so I hope am clear and it helps