30 Degree Angle to the Boom
From what I've read, the optimum angle for a vang is 30 degrees measured from vang to the boom (60 degrees measured to the mast). There was quite an extensive article on this in Latitude 38, a San Francisco-based monthly sailing publication, a few years back. I don't remember the details but the article was complete with a lot of geometry and physics. Based on other things I've read, some say 45 degrees. The angle isn't that critical, so if the geometry of your boat doesn't allow for 30 degrees, get as close as you can.By the way, at 30 degrees, the distance from the mast to the mounting point on the boom will be twice the distance from the boom to the mounting point at the base of the mast.