How to force Time Machine to back up to a smaller drive

[akadmon]

I recently moved my system files to a larger/faster drive on my Mac Pro and wanted to use an older 250 GB internal drive for Time Machine backup. To my dismay, even though the system drive has only about 120 GB on it right now, Time Machine refutes start the backup because it claims that 300 GB+ are required. I hate to be reformatting the system drive again just to reduce the size of the system partition. Is there no way to force TM to back up to drive which is physically smaller? I mean -- yeah, I know that at some point TM will run out of space, but I'm not there yet, not by a long shot! Why not let me deal with it then, Apple? I'm a big boy, I can handle it.