To accurately measure field cover level would require all the pasture being removed from the feild at just above ground level. A method of calibration has been drawn up to help assessemnt of sward cover.
This factsheet gives you details on how to take a sample, weigh it and then dry it before using the results and the ready reckoner table to give you a field cover.
You are able to download this factsheet from the grass+ programme. The full programme is available via post from this link.