Jon Glas has requested that I give a bit more background information on how I have been calculating probabilities. This area of mathematics is known as combinatorics, and with its relation to 40k one of my favorite areas of mathematics.
In the first video I talk about how to set up a probability problem using combinatorics. I spend quite a bit of time talking about how to find the number of combinations, and I give you four different ways of making that calculation. Note I do not have any specific references in the video to Warhammer (I do want to reuse this video in my classroom), but the 1/12 probability is the chances of a space marine doing a wound to another space marine.
In the second video I go through a couple of different types of example problems. The first problem is calculating the probability that four space marines will do exactly two wounds to another group of space marines. The second problem is calculating the probability that the same group of four space marines will do two or more wounds.
If there is something that does not make sense in either one of the videos please let me know. At some point these videos will be used for instruction with my classes, and if you all have questions them I'm sure my students will have the same questions.