New studio, room dimensons, sketchup attached

Hi John and everybody. My name is Cesar, producer, musician from Brazil. Pretty incredible the wealth of information on this forum. I've been learning a lot. I do know plenty about bass traps, insulation and all that since I have made lots of them for my current bedroom studio (including 2 pairs of supperchunks). However, my first child is coming this February and that room will become his bedroom :P I'm about to build my first studio outside of the house, primarily for mixing and small recording sessions. I don't need any recording booth as this would decrease too much my room dimensions. Recording would take place in the same room. The space I have available is irregular and those dimensions contained in the sketchup project are my limits, EXCEPT for the ROOM HEIGHT, which could be projected according to room modes calculations (know about modes, just don't know how to calculate them in this situation). The entrance door is also something that's not changeable. I have collected many projects you guys made available, two in special, which john made available called "small studio" and "components" which are great starting points. I intend to build speaker soffits as well for my Yamahas HS80 using John's project as a reference but since my walls are not paralel, I guess some things should change. Another important info, the place is noisy. Studio will be constructed in the backyard of a house that's in front of a busy avenue. So tips on wall width, insulation and drywall would be very welcome. So, before I go on with my design, could anyone tell me if these dimensions are suitable (and what would be the ideal height)? From this basic info I will start my design. Well, I guess it's all explained here. Any help is welcome! all the best, Cesar P.S. Its a DAW based studio, no console, just 1 rack of outboard that could fit in the desk.