creating a parametric family for a window bench seat

[jledgewood]

I am trying to create a window bench seat that has 45 degree angled benches of the base length fo the bench. I would like it so I can choose the lengths of all three sections of the bench to match the building walls where I am placing them

I cannot seem to get it correct when creating the family. when I change the length of the angled piece or the base piece the width seems to change as well.

any advice would be helpful