I have good news and bad news.
The bad news: I left the measurements I took last night at home >_<.
The good news:
1) I have a much better idea of what is going on.
2) I fell asleep in the hammock while testing it out last night. It was just that comfortable.
From what I remember, the trees are about 22.5 feet apart according to the measuring tape. My tree straps were hung at 6' and 6'2", so we can assume 6'. My SRL is 119" unloaded, and very close to 120" under load. IIRC, my suspension length on each side was about 80" and under load, my ridgeline hung 42" off the ground. My butt was 12" off the ground. That gives a vertical distance from tree strap to ridge line of exactly 30".
When I compared those numbers to what I saw on the calculator, I found that my tree straps were 1.5-2' too low and my suspension was about 6-8" too short on each side. That explains some extra tension, but not enough to stretch the tree straps a whole 5" each (I was able to get a helper to measure that part again).
I just now put those numbers into a calculator and came out with something interesting: that puts my angle of hang at just over 22*. With my body weight (190lbs) That puts the tension on each line at ~250lbs. Which is only 60lbs more than a hang angle of 30*. So yes, it is a tight hang. But it isn't nearly enough to put excessive stress on the suspension.
60lbs shouldn't be enough to go from no noticeable stretch to five whole inches of stretch. So the question remains: What material should I replace my current tree straps with that take 250lbs worth of tension without stretching too badly and still withstand abrasion against tree bark?
Recovery straps maybe?