Many plasterers will have their own idea of "the perfect finish". Some would even use the back of a shovel, and still call it perfect.
I think the perfect finish nowadays is nearly impossible, especially with the modern finishing plasters that set hard in a short time. Going back again to the days of siraphite (with a little lime in), you could trowel that up the following day, and even get cream off of it,, beautiful stuff to work with. Looking at some of the finishes on old lath and plastered walls (lime),, most of which were never painted, just papered and stripped many times over the years, the finish on a lot of them is beautiful, even to this day. They could work with the plaster then,, it didn't just "set up" like plasterers of today,, ( excluding lime plasters). A lot of the plaster finishes today are hurried,, you're working against the clock, and if you don't keep up with the flattening/troweling,, the plaster will get too far ahead of the plasterer, and before you realise it,,, it's set,,solid. Then,, the more you try to get rid of any blemishes, the more you polish up the surface, showing them up even more, while making the painters job a nightmare. I think the perfect finish, is when you've cleaned up,, washed your trowel for the final time on the day,, cast your eye over the work you've done, and think,, i'm happy with that,, i could not have done that job any better.