I have developed an android app (free) that can allow you to calculate the probability of successfully rolling a number of wounds (to hit, to wound and armor saves)
Here is a link to the app (https://play.google.com/store/apps/details?id=com.nagastudioprojects.victor.diceomnibus&hl=en)
The app is designed to be able to work with any game or dice rolling formula.
for wh40k you will want to set the success dice to the following settings:
Die Type - 6
Target - 4
Twinner - 7
Twinner - 0
Negator - 0
Reroll - 0
Reroll Limit - 1
And then un-check the Reroll - Or Higher checkbox
For an example lets say that you have a unit with 10 attacks that hit on 4+, wound on 3+ and are firing at a unit with a 3+ armor save.
You would put the following formula into the roller:
10sst3st2oL
You do this by pressing the following buttons: 1, 0, (S)uccess, (T)arget, Complex, (S)uccess, (T)arget, 3, Complex, (S)uccess, (T)arget, 2, Or L(O)wer
That formula will roll 10 dice with a target number of 4 or higher (to hit), and then roll that many dice with a target of 3 or higher (to wound) , and then roll that many dice for the armor save (3+ armor, you want to roll 2 or lower for the counted wound).
Once that is input you press the ? button to calculate the probability. Lets say you need 3 hits to kPost too long. Click here to view the full text.