I think the visually evaluation makes sense because that is the way one views a print.
Instead of comparing to the reference I simply chose the field which showed the first tone separation from black resp. white.
After all it's a personal thing what tone to choose - with this choice you set the tone you will get at the extreme ends of the displayed grey scale. Some (like me) prefer nearly black and nearly white, others want more separation and choose a very light and very dark grey.