Creating a vaulted ceiling

[ElzaBDesign]

Hi all, 

 

I am struggling to create a vaulted ceiling on the 2nd floor of house with a Gable roof. My ceiling height is at 78'' and the walls start pitching at 43'' Floor to wall break. Somehow, I just cannot have the program accepts my 78'' ceiling height. I don't understand these numbers! Why is my Floor at 121 3/4'' ? Any help would be highly appreciated :-)