This is a great method that hadn't occured to me, thanks for sharing!
Also, glad i'm not the only one who noticed Uma Therman's thumbs! It's been freaking me out for years. That and Angelina Jolie's knees.
"Nothing travels faster than the speed of light with the possible exception of bad news, which obeys its own special laws." ~ Douglas Adams
Asked TeeDee about this and he said not to get hung up trying to get the 30 degree sag angle.
But if you want, there is a very easy method for determining when you have a 30 degree sag angle.
Take a look at this image he calls a 30-60-90 triangle (that's the magnitude of the 3 angles, a being the 30 degree angle):
With a a 30 degree angle, side o is exactly 1/2 of of side h.
So measure and mark a known length on your suspension, say 6". On the upper mark, the mark furtherest from the hammock, tie a string and mark the length that is exactly 1/2 of the first length, say 3". Tie a weight on the end of the string, say a nut.
When you hang your hammock, if the imaginary line joining the two end marks is level, then you have a 30 degree sag. You can eye ball to see if the joining line is level, or you can get more accurate and use a carpenters level.
As he said, don't get too concerned about trying to get exactly 30 degrees, but then if you're laying in the hammock thinking about it, it might be a good way to go to sleep, Probably better than counting sheep.
The 30-60-90 degree triangle is a draftsman's tool, or was, when there were still draftsmen. Once you've used one for a semester , the 30 degree angle is intuitive.
It may not be rocket science, but it's still science.
Although in retrospect, I really like oldgringo's thumb and index finger method.