Sweet spot is definitely the way I look at it - my gap feelers have my Schmidling (JSP) malt mill tuned at .040", which I prefer. That is pretty course for many people, but I tend to mash for 90 minutes and get mid 70's efficiency with that gap. Sometimes a particular maltster might affect that, but not all that frequently nor all that much. I am not a stickler for eking out that last bit of efficiency, but appreciate that others may wish to do so or just hit their sweet spot at a tighter gap.
Cheers.