Technology progression has allowed divers to calculate safe limits and NDLs using dive computers. The big advantage is that the computer stays with the diver and adjusts for depths continually. If you stay shallow the computer gives you more time underwater. If you go deeper, the time underwater is limited.
To plan with the computer, you will need to consult its dive planning function. Ask your instructor, or read the user manual for your computer. Once you get into the dive planning mode, you will be able to put your planned depth in the computer and see what available time there is for the dive you want to do.
The images below show the available dive time or NDL for 12, 15, 18, and 21 meters (40, 50, 60, and 70 feet). As you can see, the dive time displayed in the middle decreases as you enter the deeper depths.
Let’s suppose you are planning a dive for 21 meters/70 feet for 40 minutes. Looking at these examples, you could not do the dive. Why? The dive of 21 meters/70 feet is too deep for your level of training, and the time is outside the NDL for that depth. The best option is to go back to the plan for 18 meters/60 feet, which gives you a maximum of 52 minutes according to this example dive computer. This means you are inside the maximum of 20 meters/66 feet and inside the NDL.