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 kill off the unit, you then input 3 and press 'Or Higher' and it will give you the approximate chance that you will roll 3 sticking wounds or higher ( in this case ~9.2%)
Please let me know what you think, also if you have specific examples that you want to know how to input please let me know, because the app is an omnibus for nearly all integer based rolling systems working it out for wh40k takes just a slight amount of familiarization.