It might look a little sloppy but it works. You can use this Java script program, swiped from www.packetradio.com, though it can be found a few other places as well to calculate the sizes/dimensions of the various parts of your J-pole design. yeah its probably bigger than you thought it would be. Many of the dimensions are very similar, so if you are looking to be a little sloppy calculate several frequencies and use the average. You could also try making some of the copper piping slightly larger to allow it to slide over itself and play with tuning that way.