Uhm, I use a template of a Connie Nacelle personally, but keep in mind the Nacelle gets smaller as it reaches the back end. It's only really noticeable side-on though, so... 4/5th's the size of the front end maybe? When I model them, I usually build a mould/stencil thing to boolean off the back so it's nice and smooth and still has that S shaped ending. (To which I owe many thanks to Limey for reminding me to merge everything after that step.)