a play-progress bar should be tied to the frame numbers of a movieclip, that a time.
That is because getTimer() measures the actual elapsed time since a given instance of a player was started. It doesn't measure the time on a given frame.
If you run the same test over an over you will see that the time on a given frame fluctuates by a little bit. Depending upon what else the processor is doing and....for all I know the phase of the moon. FLVs are different and they actually have time information embedded into their structure.
However if you know how many frames your external movie has and you know its frame rate you can "estimate" the duration.
duration = total frames * fps
current time = current frame * duration
However it will just be an estimate. And on long clips it could differ significantly from a getTimer().
So I would recommend not using getTimer at all and always just faking it with the esitmate.
Totally messed up the math. (forgot my unit analysis!)
duration = total frames / fps
current time = duration * current frame / total frames