theoretically, the "eye ball" method is best if you are trying to match the curve of your mainsail. Unless one is sufficiently able to measure the curve exactly (smacks of higher math and such.... and math and I never got along). I do know, however, from eyeballing, is that my mast has a slight twist in it. Maybe 1cm rotated at the top compared to the bottom. (crap, there just went my resale value