Your data won't work unless for each person in X is given a name as well. If my LS mate and I both post we know 3 people it is possible that we know the same 3 people. So if you were to do this properly you would need to have a list of names, remove the repeats, tally up how many people are on the list, and then divide by the number of people who posted to find the proper value for X.
Also your formula would have the be:
P + (P * X) + B - C = N
This is because yeah you are adding the people we know of that will buy the game and the unknown population as well, but you are completely leaving out the current playerbase in the final number.
To help the thread, I currently know 0 people. I will get back to you if I remember any.