Hey ho.
I'm one of the "wonks" that did the optimization math. (It's fun!)
In play, it's true you have to guess the AC of the opponent.....but frankly, that's pretty straightforward. And then you have (printed out in bright, cheery colors!) a graph that shows your optimal PA for each AC. Quite helpful and speedy.
But even this is probably unnecessary; knowing the range of probable ACs your opponent has gives a corresponding error on the PA guess, so it's usually safe enough to estimate, which can be done in a second or two at most.
Example:
"My barbarian charges at the Dire Tiger, and swings his sword. Mumble: (I guess the Dire Tiger's AC is ~ 18, so I power attack for 5.)"
"Hey, hey! I hit! ....and I did 44 hp of damage! ...Err, it's not dead yet, is it?! Oh."