Depends on how you define the probability of getting an xx-cross. The chance of getting an LL skip with no influence is 1 in 15552. If you define getting an xx-cross as having 2 F2L pairs solved after you complete the cross, assuming that you didn't try to influence the pairs at all, the probability is a little less than 1 in 18816, but surely there is some influence here that significantly increases the probability. This would make the desired probability about 1 in 300 million.Chance to get an xx-cross with an LL skip on 3x3?
It's not complex math; it's right there in the post.