Technology progression has allowed divers to calculate safe limits and NDLs using PDCs. The big advantage is that the PDC stays with the diver and adjusts for depths continually. If you stay shallow the PDC gives you more time underwater. If you go deeper, the time underwater is limited.
To plan with the PDC, you will need to consult its dive planning function. Ask your instructor, and read the user manual for your PDC thoroughly. Once you activate dive planning mode, you will be able to put your planned depth in the PDC 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 planned bottom 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 PDC. This means you are inside the maximum of 20 meters/66 feet and within the NDL.