I am of the opinion, thumping the shrouds with a rubber mallet as posted previously on this site to determine the tension is a load of crap. I downloaded a manual (and there are quite a few) on masts and rigging from Seldens Rigging and Masts site, in particular "Hints and Advice" manual at http://www.seldenmast.com/frameset.cfm?id=6828, There is a method on Tensioning the cap shrouds on page 28. I downloaded this PDF to my iPad and saved it to access in my Reference folder in iBooks.

Best Regards
Barry & Robyn
Tradewinds III SM 171 (currently Bundaberg)