If you'd like to use an actual CAD program, you can download a FREE version of DraftSight for either Mac or the PC platform. If you know anything about drafting, the guys that make DS also make Solid Works, a top notch 3D program.
Anyhow, it's comparable to AutoCAD, but w/o the steep price tag and/or educational requirements.
http://www.3ds.com/products-services/draftsight-cad-software/free-download/
Also, here is a website where you can enter your rough data and it gives you some quick & dirty info on the fly. It's not taking into account ceiling joist thicknesses, roof truss thicknesses, etc but rather giving you raw dimensions.
Scroll towards the bottom and calculate the ridge height. Be sure to subtract your wall height it will show you the "raw" height of the attic at the ridge point. So if it shows 13' ridge height w/ an 8' wall then subtract 13-8 = 5' is the actual attic height (again, w/o consideration of lumber thicknesses, etc).
http://www.pole-barn.info/roof-pitch.html
My previous house I built I had to take all this stuff into consideration. I built what they call a 1.5 story around here. It's basically a single level home w/ a high pitched roof that allows extra rooms upstairs. In my case, I added about 800sf game room & bath (above) to my 2300sf home (below). But my house had 10:12 and 8:12 pitches on it. Like your initial sketches, I used 5' pony walls and followed the roof trusses up to about 9' ceiling height and then I flattened it out. Around here, that's called a "french vault". The advantage was it still allowed some room to run air ducts for heating & cooling, plus some can lights, as well as adding some architectural interest.
FYI, the 5' pony wall was a minimum requirement for my area to be considered living space. I've seen some people start lower at 3' or 4' but that wouldn't work in my case. You might check in your area too if that is a concern (might be for future resale value).
To me, it looks like you will need to extend overall height your exterior walls (but hold down your interior ceiling height. Or run the wall heights you want and increase the pitch of the roof. Or possibly some combination of the two to achieve what is aesthetically pleasing to you, and works best for your budget.