According to the rules cover must be checked against every miniature in the target unit. So for example if an AT-St is firing at 5 men squad, 3 of them behind a building and 2 in the open, as there are more miniatures in cover, this unit would have cover 2, but as there are only 2 soldiers in LOS in fact we have an AT-ST firing against a 2 men unit (the other 3 cannot die because there are no LOS to them) in the open with heavy cover! Is that right?

Just now, Lannes said:

According to the rules cover must be checked against every miniature in the target unit. So for example if an AT-St is firing at 5 men squad, 3 of them behind a building and 2 in the open, as there are more miniatures in cover, this unit would have cover 2, but as there are only 2 soldiers in LOS in fact we have an AT-ST firing against a 2 men unit (the other 3 cannot die because there are no LOS to them) in the open with heavy cover! Is that right?

That's right. Pays to hide behind a building, doesn't it? Yes it's an oversimplification but we are seeing this game is a hybrid of realism and simplified gameplay. Unfortunately common sense creates a dissonance with simple rules sometimes, like in this case.

Yep, and that matches the example on RRG page 22. Minis out of LoS still contribute heavy cover to the "half or more" calculation.

But it could be easily solved changing it so only minis in LOS be counted for cover... playability would be the same and it would be more realistic.

2 minutes ago, Lannes said:

But it could be easily solved changing it so only minis in LOS be counted for cover... playability would be the same and it would be more realistic.

I agree, but what you're saying is that by adding an additional rule, it would be more realistic. FFG went one step further towards simpler rules than realism in this case.

EDIT: sorry if my answer is unhelpful, not trying to be snarky. I just wanted to clarify the rule and offer a potential reason they arrived at that decision. I'm sure there we will have a handful of issues with the game we will have based on our preferences!

My point is that simplicity would be the same

In a real situation, you wouldn't have four guys behind a wall while a fifth stands in the open checking Twitter. They'd all be in motion, moving around, ducking out and back behind the wall. The rules as they are represent that.