In any quadrant over 1m you're better off averaging samples. Take a look at this example from the course manual:
If you're getting close to 50% coverage, or more, it can help to look at how much sky you can see or how much ground you can see, then work backwards from there to get the amount of coverage. When averaging make sure that you not only average the overall area but also the individual plants because they may look like they take up more room than they actually do. Keep in mind that averaging is an estimation. Get as close as you can, practice with pictures to hone your estimation ability, but know that it isn't an exact science. There is no equation to follow to calculate coverage.