The structural or adjustable structural ridgeline has just one structural purpose.* That is to set the arc or hang. Now, it is common to also refer to a tangent to the arc, usually at a point further up to the tree,so the ridgeline can also be said to set that angle; but it is the overall length of the arc which is being set by the ridgeline.
Draw for yourself an arc, a broad V shape on a piece of paper. Nothing says the hammock has to occupy that whole arc. In fact the hammock must be hung so the ends of that V will usually be occupied by a rope. My point is that the % referred to, something between say 80% and 90% can be the taken the ratio of a horizontal line intersecting the legs of that V just about anywhere. (I made the usual "U" a "V" so that the previous statement would match visual intuition about it.) If you want to run a ridge- line between your tree-straps, that's OK, too
Finally, the 83% figure most often referred to here is not magic, and does not result in the hang angle being 30 degrees. It is a few degrees off, about one degree for each % point.
Refer back to that broad V shape. What you are doing, assuming you were jack-knived in the hammock, is setting the ratio of one half the ridge-line length to one leg of the V, and the ratio of one leg of the right triangle to the hypotenuse. Commonly said to be "setting the sag."
* (Yes you can hang pictures of your children, siblings, or unrequited love from it, too; but that wouldn't satisfy a structural purpose.)