Mine's running at about 71% this month (4k hands, 43 of 60 folded), basically I've a decent idea of knowing when i'm beat. I have no idea what figure is optimal, even if there is an optimal or average figure. A high figure at least means you're not a showdown muppet, but too high might mean you're too weak or getting the wrong type of hands to showdown.
I think we need excession here for his take on it

where is this number found?

It's the 3rd heading from the bottom on my PT stats. First go to "more details" on the far right of "player summary"

mine is 76%, 66%, 66.66% at 3 different sites. looks like around 70% is average