It seems to be how much your body needs for its condition, which would be hard to truly understand. It’s not the “one size fits all” it was believed to be.
If your health is poor, you could benefit more from more normal sized doses. People with COPD appear to need fairly more, to help with some healing, for instance.
Younger people don’t seem to need it much, or at all, however, their primary antioxidants and processes are still fully functioning.
The most affordable and convenient NAD precursor is NMN (nicotinamide mononucleotide). I take 1g (1/2 tsp) in powder form under my tongue before food in the morning since it breaks down with digestion. There is also a capsule form if you prefer. You probably don’t need it until age 45 when your natural levels decline.
I feel more energy during the day and sleep better at night. Other people report better performance in sports and exercise. In the long term, the goal is a longer life without mental decline.
Amazon has sold untested and mislabeled NMN products in the past. I would only trust an independently tested source from DoNotAge.com in the UK.