Hi
As you say correctly, hitting L will split the clip at the red line but if you have positioned this correctly I can't see why you are getting a difference of (1/2 second,)which does seem a lot - about 15 frames. It might be more accurate if you are "zoomed" in on the sequence with the slider bottom right of the screen. This gives a little more accuracy to your split position when you use the step buttons I mentioned above although I can't see even this correcting the difference you are seeing. Normally the film splits at the frame previewed in the right hand screen.
One thought....What reference point are you using to cut your clip? Is it visual decided by the image shown in the right hand preview window, or is it audible, decided by the hearing the cue in the sound?
Visually, I can't make it split anywhere other than at the frame I want. MPEG4/MPEG2/AVIs all seem to split where I want them.
Nat