Line of Sight, looking for clarification!

[John Fisher 764]

So, I have seen the message board for line of sight and I did not see any clear answers on the rule. My group right now is having a real problem with this and has caused quite a few rage quits. The most common scenario is when ...

[ ][x]
[ ]|
[ ]|
[ ]|
[f]|

the Fighter is trying to hit the x with a range attack, but the x is on the other side of the wall with each [ ] being a 5ft. square.
with the cover rule they way it is now,

"Cover
To determine whether your target has cover from your ranged attack, choose a corner of your square. If any line from this corner to any corner of the target's square passes through a square or border that blocks line of effect or provides cover, or through a square occupied by a creature, the target has cover (+4 to AC)."

the way I am reading this is that as long as you can draw a line from one of your corners to an opponent's, you have line of sight. But I am having an issue on how far back before the fighter can not see the opponent. It just seems that if you draw line from your corner 1 to his corner 1 that you should always be able to have line of sight in this scenario.
[1][2]
[3][4]

[Geistlinger]

X has cover from the fighter's range attack.

If you use fighter's corner 1 as marked in the diagram.

.
[1][2]
[3][4]

A line traced to X's corners 2 or 4, it passes through the wall. Thus, X is considered to have cover.

For X to be completely out of line of sight though, it'd have to be one square further back.

[ ][ ][x]
[ ]|
[ ]|
[ ]|
[f]|

[Thazar]

While the rules are a simple simulation. One other thing to keep in mind is range. If that fighter is trying to throw a dagger, he will not only suffer the 4 point cover bonus buy an additional 2 points for each 10 feet. Now a crossbow is another matter and must be really far back.

But think of it like this. Since hit points are abstract the target may just have part of a leg or arm sticking out and that is what gets hit. If the target really wants full cover he needs to take a five foot step back as suggested above.