So I thought to myself if I was going to use TeeDee's method and still reserve the right to have an adjustable ridgeline, why I'd be bothering the guy all the time to make up new tables for me. Or I could bring my HP calculator (with post-fix entry, of course) out on the trail, along with my laser guided optical range finder to nail this suspension length thing cold.
But having survived freshman physics, once, I retained that things can get simpler if you choose the right frame of reference. So I choose origin of my coordinate system to be the point where the suspension cord attaches to the hammock. What I'm clipping out of the system is the height the hammock is above the ground, because that varies to personal preference. So however it is I've figure out what the distance from hammock end to tree ought to be when centered, that's a variable that depends on site selection. Another variable is the attachment point on the tree. Given these two, you can compute what the length of the suspension cord ought to be, and what the angle of inclination is.
With this one table then you can estimate what length of suspension cord to use to attach at a given height on the tree (above the origin), or for a given suspension length what that height will be.
Lurking in the background of course is physics, that angles which are "too shallow" carry too much force on them. So in the table I cooked up, I mark table entries according to whether they correspond to angles greater than 30 degrees, 25-30 degrees, 20-24 degrees, or 15-19 degrees. Shallower than that is bad for your health and those entries are blacked out.
So I think I'll print this out, get it laminated, and put it in my hammock's stuff sack. I've included here in case anyone else can figure out what on earth I'm talking about, and wants to use it too.