In reply to ledifer:
First choice: Use the rope.
Second choice: Adjust the rest of the belay slightly to use the full sling length.
Third choice: Tie an overhand in the sling, clip the loop formed. Best done with someone else's kit as they're a pig to untie.
For very small adjustments, basically just tweaking for comfort or perhaps equalising a pair of side by side nuts to work as a single piece you can take a couple of turns of the sling around the krab.
jk