Try Geoff Moore's article:
http://www.shoresails.com/onedesign/guide1.pdfHere are a few others:http://www.doylesails.com/j24-tuning.htmhttp://www.j24fleet55.com/docs/UKJ24TuningGuide.dochttp://www.skelleysails.com/j-24_page.htmYou may also find this blog useful:http://www.j24class.org/usa/forumarchives/sailsandtuning/mastbutt.html