Hard to answer your first question. Should be adjustable in height ideally, with a track on the mast. However, many boats use a fixed ring, and if I recall correctly, the ring on a J-24 is approx. five feet off the deck. The spinnaker pole length should be 9.5 ft.(= J). Strictly speaking you should subtract the diameter of the ring on the mast from that length, but virtually no one would quibble with you on that.