Or.... boom leg is twice the length of the mast leg. (for 30 deg.)
If you're setting up a rigid vang... then the hypotenuese becomes the determining factor....but a soft vang, it's irrelevent.
As jackdaw mentioned, the height of the boom above the deck may require a modification of this formula..... for instance: a 10ft boom rigged 3.5 feet above the deck would place the bale 7 ft out on the boom..... I certainly wouldn't go past half way. So to maintain the desired ratio, you might have to raise the mast attachment point.... of simply settle for a larger angle.... 45 deg would be my maximum...
Another one of my idiotic points is this....don't forget aesthetics.... if you have a dodger, match the angle of the vang to the angle created by the dodger's front window....