20/20 is perfect eyesight; to have a perfect perception of something is to have 20/20 on it.
Another expression is "hindsight is 20/20/" That means to look back on the (past) situation now, you can see it perfectly.
I never should have bought that car.
Well, hindsight is 20/20. You didn't know then that it had been in a wreck.
- For Teachers