To add to this, I'm not even sure that pros have a definitive count in their own mind. I remember watching Michael Redmond do a game commentary and he commented that one side was ahead. Someone asked by how much and got a fairly generic answer in response ('quite ahead', or some such).
I piped up and asked Michael how many points komi he would want to switch sides and that stopped him in his tracks. He looked at the board for a good minute before coming up with a number.

Commentary of 4th game by An Younggil at GOGAMEGURU.


http://gogameguru.com/go-commentary-gu- ... go-game-4/

When White created a thick shape with White 26, reinforcing the bottom area with Black 27 became necessary. The result up to here was even.

After the game, readers at Go Game Guru discussed this position on our website and many people thought that the result was better for Black. The result does look good for Black at first glance, but most professionals, including me, concluded that the result was even.

There are a number of subtle factors to consider in this position:

1. Black has to defend at Black 27, so White takes sente.

2. Black started out with four stones in this area, whereas White had only one. So we should expect Black to profit more than White does in this part of the board.

3. Even after Black 27, Black's position at the bottom is still weak, because of the power of White's thick group. For example, it will be easy for White to invade at A later.

4. Because Black's still weak at the bottom, White's lower left corner is actually much bigger than it looks. Black can't expect to be able to invade at B without provoking a severe counter-attack. And White can enlarge the corner in sente, by harassing Black later (see the variation for one example).

5. Because White's lower right group has good eye potential, it's much more powerful than a simple wall would be. It's more than a wall - it's thickness. Extending along the bottom is less important for this kind of group, because there's no urgency to create a base (think eyespace) for it.

6. Even though Black 27 reduces the potential of White's thickness, Black doesn't have much potential to speak of either. White's thickness turns the bottom into a no man's land, where neither player can expect to make many points. And the lower right corner is settled, mostly along the third line. Black's stone at C is slightly over-concentrated.

7. White has potential on the left side and at the top, which he can develop with sente.

8. Both players exchanged one stone (D and E), but White D still has some endgame aji, as we'll see later, so Black's lower right corner is a bit smaller than it looks.

6. Even though Black 27 reduces the potential of White's thickness, Black doesn't have much potential to speak of either. White's thickness turns the bottom into a no man's land, where neither player can expect to make many points. And the lower right corner is settled, mostly along the third line. Black's stone at C is slightly over-concentrated.

It has just occurred to me that White's group is actually a kind of "Go Seigen group" and this is a Chernobyl area (see "The Mystic Emperor, Go Seigen" in The Go Companion p. 93). So the theory for Yi Se-tol's play exists already!!!

I am very curious about Robert Jasiek's opinion.
Below, An Younggil's commentary after move 27 from the link above.