1/3rd's is just one of several different ways to plan a dive. Honestly you should use a gas plan compatible with how you want to end the dive.
Do you
need to get back to where you got in, is it merely preferable to return to the starting point, or does it matter at all? Using 1/3rd's all the time would significantly decrease bottom time while adding a ridiculous safety factor to many dives that don't need it.
A better solution would be to know a rock bottom pressure for the max expected depth, and work out what kind of gas plan to use after that.