The lengths in this picture are correct. The two shift housings should be exactly the same length, cross and leave a nice ellipse shape within the head tube when viewed from the front...http://www.flickr.com/photos/mikesbikes/6668682635/
...Although I prefer both brake cables run in front - looks cleaner and more traditional to me.
For the rear brake just rotate your bars as far to the drive side as they would ever need to go and then cut to length. Then you know it has just enough. Ferrules add a little bit of length too.
In my professional opinion a standard coiled brake housing MUST be taken to an angle grinder (or at least a file) to produce a perfectly flat surface to mate with the ferrule/ stop. This prevents the primary cause of cable "stretch" as well as decreases friction in the line. Every high quality bike deserves this treatment during buildup, however it is almost never undertaken. For truly precise install, an angle grinder/belt sander is necessary.
I have a special technique that produces a housing end almost as flat as the factory cut, and have found that this trick can yield huge gains in shift/brake performance with all of the major groups, as well as less need for adjustment since the housing is less likely to collapse within the ferrule over time. Obviously this does not apply to your shift housing as you are using segmented housing.