Storage area (2nd floor?) in shack

[65Shelby]

I cant figure this one out...
I am buliding a little shack and was going to use 2"x6"x10' for my walls but keep ceiling height the same at 8ft.  Allowing about 1.5' remaining on the walls with a vaulted ceiling in the "attic" or storage area.
I've tried two methods; the 10ft studs I stated above, as well as 92 5/8' studs with a second floor and shrinking the second floor walls.  Neither work for me, they always show the ceiling at the joint of where the wall meets the roofline.  Hope these pix help...