Last I checked, Omniweb doesn't provide a way for me to detect when it's done downloading. I've sent them a request but have not heard anything from them as far as anything being implemented. You may want to bug them about it to cast your vote.
You could also add a condition to your rule to not process the file unless Date Modified > 10 minutes or something to that effect. Not ideal but something to consider as a workaround for now.