Jim, when I first started hammocking, I used those pocket 'net' hammocks from Walmart. Those things need to be hung very tautly just to be able to lay anywhere near flat.
The ropes felt as tight as piano strings.
The newer 'assym' style hammocks don't need to be hung tightly at all, and my guess is, because you are laying diagonal to the rope axis (outside the triangle) you are able to get much steeper sides to your "triangle". To make the trig work, the angles need to be measured after the hammock is loaded and the intersection would be about where your butt is. Maybe a 70-80 deg. angle at the 'butt point', vs. 30-40 deg. with the old net hammocks. Your math is right....just have to figure out how to measure an 'assym' hammock.

Now, having marveled in the amazingness of super-light beeners, have a look at the load ratings of this stuff:
http://www.samsonrope.com/index.cfm?rope=192
(scroll down to the bottom for a chart)