In a few weeks I start the reroofing and while reroofing I will extend out the gable ends of two roofs so that I have enough overhang to fit 4″ of XP on the outside of the walls below.

One extension will be limited to 5 or 6″, and the other can be anything I like (probably 16″).

There are two ways that I know of to extend the gable:
Both are illustrated here:
http://tinyurl.com/ldqqzj

1) Barge rafters (which attach blocks to the end rafter and build out).
2) Lookout rafters (which notch the existing end rafter to fit a 2″x6″ or 2″x4″ to extend out the roof)

Is one method accepted as better than the other?

My current plan is to use belt, braces, and a piece of string (at least for the 16″ extension).
That is to say, make a lookout rafter, but fit the blocking too, with the blocks fitted with steel hangers on the old end rafter, and spaced between the lookouts.

Also for the lookout, is it best practice to bring the lookout in one additional rafter?

Also if using lookouts because the existing end rafter has been notched and weakened, should one sister a smaller rafter to it (i.e. if existing is 2×6 sister a 2×4). The rational is that the smaller sister won’t have to be notched and strengthens the weakened rafter.

I do have a professional roofer to do this, but I am working very closely with him (50% of roofers weren’t interested in this job).