If someone shot 100% of their field goals from 3, then a 40%fg% would be an elite shooter. You can slide the scale however you would like, but this is kind of the whole point and why we have numbers like efg% and ts% that should always be used when comparing efficiency. I suppose if you have the fg%, 3pt fg%, and %fga from 3, then you could compare, but it's just so much more simple to use efg% or ts%, that I'm not sure why anyone would do anything different.
Again, not trying to be a dick, but your responses make me unsure if you get it. There really isn't a good reason to use fg% that I can think of.