I put some mafac racers on my nishiki mixte, and the rear straddle wire is about 120mm long (which is rather generous--there is extra unneeded length). The racer straddle cables don't have double ends, so a derailleur cable may be cut to size to work. The length though may be specific to your frame since the brake bridge may be shorter or closer to the seat tube on your bicycle than mine. You only need enough cable to travel around your seat post and reach either side of your brake. Why not measure how much clearance you need on the bicycle itself?