Welcome aboard! I'm sure you're finding as many helpful folks around here as I am.
I had the same problem when I tested hung my BB last weekend. My trees were too far apart (about 20' or so) and my stapps were connected too low, so when I adjusted tension to get the hammock up to a comfortable sitting height the RL was tight. As Brandon (Warbonnetguy) pointed out this caused the hammock to stretch down and sink down as I got in. Brandon gave me the following information on my similar thread a few days ago:
"here's an illustration, your blackbird has different dimensions so the numbers won't be identical, but the idea is the same: http://www.hammockforums.net/forum/a...5&d=1219329859
Good luck and have fun.