First I need to say that all of my rods have at least 900 feet of line on them as a minimum. The larger fish we have over in Lake Ontario love to run more than 600 feet. That being said, when I have a fish on of the caliber that you are talking we generally start a turn and start clearing line. This does two things. First it puts a different angle on the fish which tends to change its direction of travel and second you will either gain line or find the amount he/she is taking will slow to a stop. Not all fish are the same, and not all circumstances are the same. Other boats and depth change may dictate another action. Some may agree and some may disagree but this is what has worked for me every time I have a true screamer on that is approaching the 600 foot run mark. Good luck and tight lines.