The 83% is just a ball park (generated by some of the physicists over at HF) starting point to work from for you. Most folks find that if they play with the lenght of the ridge line they will find what works for them personally. It can also depend on the hammock a lot in how much sag to put in and I know a couple guys that are now hanging some styles with a ridge line of 50% or less. I also know a few that like their hammock drum tight and would be working with a ridge or 90 or higher.
The 83% is only a starting point, don't get to caught up in what the exact number is but focus on the comfort of the hammock. Like Tim mentioned an adjustable ridge line can help as you can make little adjustments to find where you get the most comfort for you in a particular hammock.
Suspension angle and tension DOES play into the ridge line's effect on hammock comfort. Having a ridge line dose not mean you can't over tension the suspension or under tension it. It only is there to assist as a guide in set up. You still have to apply the correct set up practices to the hammock suspension lines for the hammock to lay correctly. This comes with practice and experience not by set it till the ridge line is taught and forget the rest.
Most folks do find that setting your feel slightly higher helps to prevent your upper body from sliding "down hill" and leaving you scrunched up in the bottom of the hammock. It only takes a very slight angle to get your upper body to to be the lower center of gravity so you don't slide. If you still then like to have your head elevated there is a good place I know to get pillows made specifically for use in a hammock. :)
Good questions. :)
