Well, if you believe in Cartesian skepticism (that is, that we might be in the Matrix), then your probability for anything can't ever be zero. Say the probability that this world is an illusion within another world is epsilon: in that case anything could be true. So the lowest probability we can assign to anything is epsilon. EY has a post on how 1 and 0 aren't probabilities.

Find the probability p(A | B) where B = "nothing weird like what the Cartesian sceptics talk about is going on". That gets rid of the Matrix issue. Then it is just a matter of whether or not you want to let dart players divide by infinity all willy nilly.