I think we are missing the point here This is a novice diver. She is doing her first deep dives. Diving from a boat the briefing should include an amount of air they want you to come back on board with. Same for a shore dive. Subtract that amount from your supply whatever is left divide into thirds. Wreck and cave divers use this as a rule of thumb and will work well for you. Assuming a 3000 psi tank and on-board of 500 psi you have 2500 to play so divide by 3. Gives you 833 psi for each phase of your dive. down, around and back. These are estimates and your numbers will differ. Do your own math and remember you can call your dive at any time. Check your tables to ensure you are staying within your NDL limits for the depth and time. Relax, and enjoy. if you have to err do so on the side of caution. Plan your dive, dive your plan. Hope that helps