Hiya
The international rule is regarded as a 1/3 of your breaking strain. Specifically for boat anglers who are targetting large game-fish with lever drag reels. Why 33.33% then? Quite simply, knots weaken line. Secondly, when you set your drag, your reel has a full spool of line. Should a fish take half you line off your reel, the reels diameter has now changed and your drag setting is actually much higher!! I did this test when i was teaching my crew about loosening your drag when you spool is half empty. I set my drags on my lever drag 50W's to 8kg's. When the reel was half empty, we checked the drag and it read over 12kg's. Filled the spool up again and it was back to 8kg's!! Thirdly, with plenty of line out on a big fish, the DRAG of the line through the water has also to be taken into account. Boat anglers will know what i'm talking about, throw out half your line behind the boat, with nothing attached to it and try and reel it back in!! HARD work!!
How-ever, this is not really applicable to rock and surf anglers. Most simply fish with a slightly loose drag and if you hook a fish, that's too small and doesn't need drag, you simply tighten up the drag.
Why isn't the above applicable to rock and surf anglers? Well, lets start off with this scenario. You get a puncture and have to remove your cars wheel. You take out the wheelbrace and try to loosen the nut, but it won't budge. So, you take a long metal pole and slip it over the wheel brace, basically making a powerbar and then you can easily loosen then nut. Now, the EXACT same principle applies to fishing. YOU, the angler, are how-ever the "NUT" on the wheel!! The longer the fishing rod, the more pressure a fish can exert over YOU. Thats why boat anglers, fishing for very large game fish, use short rods, for more leverage.
This in turn means that long surf rods can't pull heavy drags. If you were fishing 18kg line (.52mm siglon), you'd need a drag setting of 6kg at 33.33%. I can't remember what my surf rods tested as a maximum for drag, but very few surf rods can pull that sort of drag!! I think it was around 4kg's. Would actually be very interesting to get somebody to test their rods. Simply attach your reel, thread the line and tie to a electronic scale. Pull then as HARD as you can and get the reading from the scale.
Other than the above, fishing with too a heavy drag will cause you plenty of lost fish due to hooks pulling out!! If fishing off the surf, rather fish with too a soft drag, as the fish will eventually tire. When fishing off the rocks, lock up and PULL!!!!