You're right with SLS for your hammock, the line will sag. No idea how to get the proper angle other than by trial and error, or measure the angle of the attachment point to a parallel from the ground. Another thing you could do is keep a ridgeline attached to the two ends as a "guide" for your angle, this would be useful for SLS and traditional suspension.
No rule of thumb for the gap. It depends on your angle of hang. Some people hang at a higher angle, or if you use SLS you will need to anchor higher to compensate for the sag, etc. The length of the space between anchor trees will impact this as well. It's a bit of trial and error to figure out your specific setup.