First, you are talking about diving in a overhead environment! You should always have a topped off tank and should never dive a tank smaller than your buddy! In Cave Diving that can get both of you killed! When you are in an overhead situation you have to think of your buddy team as one unit. You have to carry enough air to get both of you to safety! It is the rule of thirds! And you are going in a third down to start! If your Caribbean diving in open water I could care less! But if your doing a 63cft in wreck dives you won't be my buddy or I wouldn't let you in the water on my boat! Take a wreck course or a cave course! You can't make more are at 80ft! For Some people a 63cft is a bailout bottle! you can always dive a bigger tank than your buddy but never smaller!