tbone, I don't know if you accounted for this in your calculations.
Also, this explanation might be useful for the casual reader:
Not all hashes are actually stored in the precomputed tables. For each "initial password", its hash is computed. Then the hash itself is taken as a new password (I think it's adjusted for the valid input chars) and a new hash calculated.
This process repeats a number of times until you have a final hash.
The only thing you store in the table files is the initial and final hash for each of these "chains", which reduces most of the space required. Still you'll end up with 20Gb of data; just imagine all hashes being stored
When cracking, the known hash is transformed with the above process to obtain a list of hashes to form a new chain of the same length as the others. If one of these hashes matches the final hash of any of the stored tables, you have found the chain where it belongs. Then you just have to find the previous hash that chain - that is the password you're seeking
