The only time I'll dive thirds is when I'm diving in a cave. For internet diving, I religiously stick to 1/10ths. I write a post, then delete 9/10ths of it. Posting more than one tenth of what I write runs me into the risk of too much internet penetration also known as caring way too much about the internet and not enough about real life.
(on a serious note, how many people actually dive something more conservative than thirds at Peacock? I mean actually dive it, not just post that you dive it. And I mean full cave divers, not intro divers, even the intro divers that dive thirds because they think they are Big Cool People that are above the guidelines.)
Most gas rules state, "Always reserve
at least 2/3 of your gas supply for exit.
I have to admit, I dive thirds - unless the conditions suggest sixths. It keeps everything simple. Every cave diver understands (or should) calculating thirds with same and dissimilar tanks. Thirds becomes the line in the sand that you absolutely cannot cross. No exceptions. No excuses. Thumbs.
If decompression is a factor, then without deco gas it's a great idea to take away rock bottom gas and calculate thirds based upon usable gas. If you drop deco gas, then you may just calculate thirds for penetration. A diver can decide how much caution he or she wants to build into any dive.
Once we've established that line in the sand, we are now free to draw upon our experiences, our emotions, the formation of the cave, the conditions, how the dive is proceeding, etc., to decide when to turn the dive. I've turned dives before thirds for all manner of reasons: time, depth, deco, multiple directional changes, silt, gut instinct, name it.
There are so many factors that can go into turning the dive. When I first started cave diving, I thought 3 jumps was a lot. After gaining lots of solo diving experience making 50 directional changes or more in a single dive in low flow, Florida caves are more easily pictured in my mental map. So, I'm more comfortable exiting in the dark. When diving with my girlfriend who easily does the Bone Room circuit to the Mapleleaf in AL80's and is a very cautious cave diver, I'm happy doing thirds with her. She has good instincts and know when to speed up or slow down.
Most divers tend to use more gas at the beginning of the dive even in low flow as they settle into the dive and find a rhythm. As divers get a little cooler during the dive, the swim pace is often picked up a bit during exit. Divers also tend to exit faster with the idea of, "Hey if we speed this up we won't have to hang as long." At this point, many divers are still breathing better despite increased effort due to the fact that their bodies have moved into an "aquasize" mode. However, if the exposure is prolonged enough to cause the diver to use more gas due to being cold, then we're back to using more gas in a shorter period of time.
If you want to go into a dive feeling a bit better, try facial submersion with no mask as you check your regulators in the spring run or basin. Take deep inhalations and very long slow exhalations and see how you feel going into something like the Ear or Eye at Ginnie.
Have there been dives when I have planned to turn a couple hundred psi before thirds? Absolutely! But, most of the time I'm comfortable with thirds. I'll admit it, Jah Jah.
