The rule of thirds is a guideline, and one should indeed adjust it to the circumstances. As a guideline, it is a starting point for planning, and variations from that starting point should consider it to be the maximum. You deduct from that to make allowance for less than optimal conditions. You also don't see it as a goal, and you should be ready and willing to turn earlier. My last cave dive was a few days ago, and we found the visibility in the cave to be terrible. We went in for a while, and when it didn't get better, we turned the dive. I was just past my first third when we exited the cave.
You need to remember that all thirds are not created equal. Andy mentioned this briefly, but if you and your buddy have different amounts of air to begin with, you use the volume equal to whoever has the least air as your turning point, meaning that the diver with more air turns earlier than 1/3.
You also have to remember that it is not just loss of air that is being considered--it is the time it takes to solve a problem as well. Let's say you have enough air with you so that it will take you 20 minutes to reach a third. That means you have 20 minutes to solve a problem (such as a silt-out with a lost line). If you are instead only have enough air for 10 minutes to reach 1/3, that means you only have 10 minutes to solve that problem--and probably less because of anxiety.
You need to remember that all thirds are not created equal. Andy mentioned this briefly, but if you and your buddy have different amounts of air to begin with, you use the volume equal to whoever has the least air as your turning point, meaning that the diver with more air turns earlier than 1/3.
You also have to remember that it is not just loss of air that is being considered--it is the time it takes to solve a problem as well. Let's say you have enough air with you so that it will take you 20 minutes to reach a third. That means you have 20 minutes to solve a problem (such as a silt-out with a lost line). If you are instead only have enough air for 10 minutes to reach 1/3, that means you only have 10 minutes to solve that problem--and probably less because of anxiety.
