for each range, do 2 to the power of the bit range. Example 2 ^64 = 18,446,744,073,709,551,616; now divide that by # of key/s. example; 2 ^ 64 / 1,000,000,000 (1 billion key/s) = 18,446,744,073 seconds = 307,445,734 minutes = 5,124,095 hours = 213,503 days = 584 years . I think that math is accurate. but basically 2^64 / 1000000000 / 86400 (seconds in a day) = 213,503 days / 365 (days in a year) = 584 years
So, if by some chance i knew the first 2 characters for puzzle number 64 then it becomes a 50 bit password hash problem right? in which case it would take roughly 100 days to solve if i understood your formula correctly