gigobyte wrote:By my guess if 10 assault marines charge 30 orks; the orks get 60 shots. out of those 60 shots an average of 10 will land, and of those 10 that land only 3-4 will wound, and of those 3-4 that wound, on average 2 will be saved. Ending in 1-2 wounds that need to be taken on the marines.
That is the best case for orks, most other armies will be hurt more that that, since they wont have that many shots to take...
Since the rest of your question is answered, I thought I might pipe in to answer this.
Overwatch is rarely actually about the casualties themselves, and more about reducing the charge distance of the enemy unit (with some exceptions, like Wall of Death or Supporting Fire). If a squad of Marines loses two men, that's a couple of extra inches that they usually have to move when making their assault, making it more likely that they'll fail. Losing one or two models before they even get a chance to attack can also be pretty painful - especially if those models are expensive power armoured models, like Vanguard Vets, Grey Knights or Khorne Berzerkers.