kwinter
Contributor
I agree with you in general about thirds, but I think there may be situations in which it is advisable to think along those lines, even though it may technically not be true. For example, if you are visiting a wreck with an upline and a strong current, you might be advised to plan your dive to thirds because you would rather not do your ascent off the line in current. That might being you to the surface away from your boat and bring more complications than you would like.
I agree. There is also the time it might take to shoot a bag and tie it off to create your own upline in case you couldn't get back to the tie in. Simple reef swim on a drift dive where you can always go straight to the surface? No problem using half or even two-thirds or more, just so that you can surface with 500 psi (even accounting for increase RMV due to stress). Something more complicated like a wreck or reef in current near the shipping lanes or involving penetration and overhead? Then a more conservative plan and larger reserve are needed. The conditions of the dive dictate the plan.