Since "N nines" for positive integers N denotes an uptime of 1 – 10^(–N), I think it's fair to analytically continue that formula, and say that 9% availability

Since "N nines" for positive integers N denotes an uptime of 1 – 10^(–N), I think it's fair to analytically continue that formula, and say that 9% availability corresponds to 0.041 nines!