It really isn't all that useful to tell someone a general rule to apply to all dives. "Turning a dive" usually implies heading in one direction for a while and then turning around and coming back the way you came. that is different from a dive to a site (say in Key Largo or on a wreck) where you may swim around for a while and then ascend. It is different from a drift dive in which you head in one direction for a while and then ascend. In all three, you have a minimum gas reserve, but what you do before that is what differs.
In general, there are three types of plans, and what you do in terms of gas planning is different for each. Here is a simplified explanation:
1. All usable: I can use all but the gas needed for the reserve before I ascend.
2. Rule of halves: I am heading one direction for a while, turning the dive, and heading back. If something goes wrong and I don't get back, it's not the best, but it's still OK.
3. Rule of thirds: I will head out and back, but it is really imperative that I get back to the starting point before my ascent.
In general, there are three types of plans, and what you do in terms of gas planning is different for each. Here is a simplified explanation:
1. All usable: I can use all but the gas needed for the reserve before I ascend.
2. Rule of halves: I am heading one direction for a while, turning the dive, and heading back. If something goes wrong and I don't get back, it's not the best, but it's still OK.
3. Rule of thirds: I will head out and back, but it is really imperative that I get back to the starting point before my ascent.