Is there a guide to understand the basic math of the game?
Well, it's very simple. Chance to penetrate = penetration value / armor value.
So small arms (1 pen) against Shock armor (1.5) have a 1 / 1.5 x 100 = 67% chance to penetrate.
An ATG with 240 pen at X range against a 300 armor tank has a 240 / 300 x 100 = 80% chance to penetrate.
Thanks, and does cover affect grenade damages? or it is just how pack your models are?
Grenade AOE determines how much damage a squad takes based on how many models are within a certain part of the AOE. The further from the source of the explosion the less damage it does. Light cover has no direct effect on grenades, but indirectly it's worse against grenades and explosives because cover makes your squad bunch up (increasing the damage caused by AOE).
Heavy cover gives -50% damage against all weapons, including explosives/grenades. But only directional. So if a grenade is thrown over some cover, it gives 50% damage reduction. If a grenade is thrown or an AOE weapon is fired from the wrong side of cover, this damage reduction is not applied. Heavy cover also does not help against direct hits from most tank or howitzer weapons because they deal 160 or more damage, which means that even with -50% damage from heavy cover they still deal lethal damage (80), but obviously it does help against indirect hits, because the outer parts of the AOE profile for most weapons is lower than 160 damage.