There are a number of general " rules of thumb " that I won't get into here { I'll let the " experts " handle that-you know who you are- LOL }.
I've found that I can judge within about 5" or so,on the hoof, and have done so through the years. Once you get above 160 or so, at least for me, it starts getting incrementally tougher-as far as within 5". No magical feat, just a lot of experience doing so.
Of course , It helps immensely looking at 5, 135 " + class every day, and knowing exactly what those scored.
Thanks for sharing that Website!! Lots of fun!