Member Jo Davies found a helpful video on the UK Sailmakers web site. The link to the Learning Videos and Articles section of the site is:
http://www.uksailmakers.com/Education/Learning-Videos-Articles.html
Click on How to Center your Rig. You'll need to register to view the video, but that is free and simple. Your e-mail address is your login.
Jo checked that it is OK for us to reference the video.
