Let’s say that you are a very safe diver and you are worried about decompression sickness, so you want to optimize your Nitrox mix. Then I am assuming that you are also worried about running out of gas for you and your partner, so you have calculated the minimum amount of gas you and your partner will need to share off each other to get to the surface safely for the depth that you plan to dive.
Let’s say that depth is 80 feet, your calculated minimum amount of gas before ascending is 30 cubic feet at a SAC rate of 1 cubic feet/minute at the surface for each diver. You dive AL80s, each holding 77 cubic feet. This means that you should both start ascending at a gauge reading of 1200 psi. That gives you 47 cubic feet to use at 80 feet (3.4 atm). That gas will last you 14 minutes before you reach rock bottom pressure and you have to go up, well before the 25 minutes NDL on the PADI air tables and nowhere close to the 36 minutes on the 32% nitrox PADI table.
To even match the PADI air NDL limit at 80 feet, you would need to bring 115 cubic feet of air. Assuming that you aren’t each diving HP120s, the limit of you diving “safely” is not the NDL, air or nitrox, but the gas in your tank.