Eric,
its the angle of the dangle that matters. If the lines hung straight down the force would be equal to the occupants weigh, and if they are perfectly horizontal that value goes to infinity, so its a matter of trigonometry. Since most people tie their hammocks pretty tightly then lay in them there is a tremendous amount of force. Lets see you would divide the weight of the hammock and occupant by the sin of the depressed angle, that is the angle down from horizontal. 180 pound load with 45 degree angle = 180/.7071 = 255 pounds. 30 degrees is sin 30 = .5. 180/.5 = 300 pounds. sin 20 = .342 180/.342= 526 pounds. Now for Bigfoot with really tight lines and non-stretch ropes at say 10 degrees, sin = .1736 so 250/.1736 = 1440 pounds.
Jim