Starting Johns 'Small studio in 3D'

Hello all, After reading for a while and looking at the many different designs here and on the SAE site I plan to start the 'Small Studio in 3D' design that John has posted. It fits my room size perfectly, room is 30' X 20' but the 20 X 12 size is what i want to use and leave the rest for storage or maybe a future live room. I have a couple of questions to either John or someone who has built this studio. The window in the back of the studio, would it make much difference if it was moved over to the left, almost touching the left hand wall? and the entry door into the control room will have to be moved to the end wall of the studio, opposite the window on the other end. How much if any would these changes effect the performance of the room? In the tracking room, is the ceiling flat, just on a downslope? On the exterior walls, i plan on using standard 2X4 framing with 2 layers of Sheetrock, for the ceiling 2X8 with the bottom covered with sheetrock, the top with 3/4 plywood, is this sufficient for the design of this studio? Im sure i will have to figure out much more than this but hopefully this will get me started. Thanks, and thanks to John for the design.

Here are some pics of where the studio will be built. The HVAC system will be moved and hung from the ceiling above the studio(not sitting on it). The shelves will also be removed from the left wall. I laid out some 2x4's to show somewhat of the layout of the studio. the ceiling height is 14'6", the only restriction is the two-post automotive lift is the second stall of my garage. from the wall to the lift is 14' the total dimensions are 30' long 14' wide 14'6" tall the studio design is about 20 ' long and that works great for the space, i have a door coming into the garage at about 23' from the back of the stall the main purpose for the studio will be for my wife to be able to play her keyboard, full size 88 key rowland, which should fit well in the tracking room. and also for vocals to be recorded in the same room.