see my previous post) each additional node adds one more 9's to overall cluster availability. That is exactly true only if the single node has only one 9's (A=0.9), which the above "Trubin"'s equation shows.
But how that would work for other single node availability numbers? What if that has two or three 9's? I have generalized my previous equation to cover that and it shows that the cluster availability number of 9's will be increasing in arithmetic progression (sequence)!