Holding the shift key you can drag the playhead to a subframe point, down to 1/100th of a second. You can mark an In point at subframe level. The out point has to be an exact number of frames and an exact distance from the in point.
All the best,
Tom
Author: “Final Cut Pro 5 Editing Essentials” and “Final Cut Express 2 Editing Workshop” Class on Demand “Complete Training for FCP5” DVD