The higher the correlation, the harder I find it to guess. (r = ... umm... :P )
I played by following my intuition and did quite well in the higher end but found the lower end much harder. I'm guessing because I was feeling my way through it, my brain was trying to see patterns even where there weren't any, pushing my guesses higher than the plot showed.
Same, although I think I overcompensated for the tendency to see patterns everywhere and generally guessed too low for R < 0.5.