What Shy wrote is spot on.
The target Nr is what the manufacturer has determined to be the compromise of most efficient with more limited protection factor. In training we teach to target that number to ensure that your reflexes will give you maximum performance, to be adjusted when you have time. If you aren't getting full droop, you aren't getting best performance. The same problem if you droop too much!
Be aware that different manufacturers use different philosophies. While an old 2 bladed Bell product generally uses a target figure of 91-ish% Nr, the Super Puma used 97% if I remember. During the conversion I was reverting to my old habits and the poor girl just would not perform!