I've read a few tutorials on rainbow tables...and I get the basic fundamental concept of how they work.  My question is...if you're doing some pen testing and you uncover some administrator accounts where you're able to obtain the first seven characters of the password but not the rest of it...is there a way to start a rainbow table cracking job where you can ultimately say "I have the first seven characters, so take this hash and only crack from character space 8 to 10"?

I'm simply trying to squeeze out the other few so I can make some best guesses, if I'm using just a simple alpha numeric table.  I wasn't sure if you can tell a cracker to only crack from this character space to this...if you already have a portion of it.