You are definitely in the ballpark. For bearing cantilevers I was always taught 3 in for every 1 out. this would mean 64", but this isn't a floor so I would probably cut up 8' or 12' studs into 4' lengths and call it "good enough". Are you going to have a horizontal soffit? If so, then your soffit framing can carry the load and you won't need more sistering length that that required for nailing purposes (one foot extra?). Also, make sure to use a TIGHT string from one end of the roof to the other to keep all your tails straight and level.