The thermal load line is a temperature curve, not a specific point like the maximum junction temperature. This isn't something that I can answer in a paragraph or two. Intel has plenty of documentation that you can use to learn about this (if you can find your way to it; their site is absolutely horrible for finding anything). There were some good articles on this in the old Intel Developer Forum (IDF) conference presentations.
I am not sure what you mean by "NUC7". There are a whole bunch of 7th generation NUC product families that this could imply and system, kit and board products within these families. You need to be more specific in your references.
That said, the curves that I defined are fairly generic and conservative and should work as a starting point for you. You will need to adjust based upon personal preferences anyway. I personally will put up with a little noise to guarantee that it doesn't overheat (which is a relative thing anyway); you will need determine what you are willing to listen to. Understand that it is difficult to cool the system without some noise, considering the confines of these chassis. You can't use fans, there isn't room. You have to use blowers and these have to spin at much higher speeds to move similar amounts of air.