3 smartie car - I like it.
A good rule of thumb is that you can begin with a 10 degree angle, and if your transition is gentle enough, that angle/tangent (if a radius) can be increased beyond that and still retain attached flow.
You could do worse than simply look at the rooflines of existing relatively aero vehicles, eg:
(from
Kardboard Kammback: testing a partial boat tail prototype )
Also, don't forget it's a 3D shape - the transition from "side" to "top" is important, and should be generously radiused to aid pressure recovery & discourage vortex formation. If you look at my kardboard prototype, you'll see I used too sharp an angle at that transition point. AndrewJ did much better with his finished work.