Planning The Dive

For those who have been diving for a short time or a very long time, you may or may not have had any exposure to gas management. A part of gas management is the management of the breathing gas in your cylinder so you don’t run out and have to share or conduct an emergency swimming ascent.
PADI recently revised their Open Water Diver course to include gas management. This is not new science and it has always been in the materials. It depended on whether the instructor understood the process, some variation of it, or flooded you mind with too much information.
Some would teach “just be on the boat with 500 PSI”, some said “use the rule of thirds”, maybe it was max depth, times ten, plus 400 PSI, or maybe you were taught nothing at all. All them except knowing nothing at all about it, works.

Here is a descent formula for determining your turn pressure or when you should start your ascent.

Find your starting cylinder pressure. I would do this after it has been submerged a period of time and not when you take it out of the vehicle.
Start Pressure: e.g. 3000 psi

Minus a reserve. Remember you want to be on the surface with 500 PSI
Reserve: 500 psi

Plan on making a safety stop and then ascending
3 Minute Stop and Ascent: 300 psi

What is the remaining pressure or the available pressure
3000 – 500 – 300 = 2200 psi

Take your start pressure e.g. 3000 psi
minus
Half remaining pressure e.g. 2200 / 2 = 1100 psi
this leaves
Turn pressure e.g. 3000 – 1100 = 1900
This is when you want to start heading to the safety stop

This works pretty well for simple dives that a newer diver may make. As your experience grows you can make some personal adjustments to meet your needs.
There are a number a good arcticles available online about other gas management stratagies.
Whatever formulas you choose to use, always plan the dive and dive the plan. Make minor adjustments and dive it again, over and over to ensure it works for your plan.